There has been much hot air expended about the future status of the UK state after BREXIT - whether or not it will be a vassal state. The harsh reality is that UK already is a vassal state and has been so for many years, not because of its relationship to Europe, before or after BREXIT, or its military / diplomatic relationship with the USA, but, because it's financial relationship with Saudi Arabia.
The modern basis of this dubious dependancy relationship is not oil, at least not directly, but the money that the Saudi's have invested and squirrelled away in the UK since before the 1973 oil crisis. The UK state as it lost its empire (and its role in a wider world) was in desperate need of financial salvation - the emphasis should perhaps be on the 'desperate'.
Pandering to world power status (perhaps at best a post imperial vanity project) rather than powering down to develop a more sensible regional North Atlantic/ European approach was and still does not come cheap. The UK states changing role, while driven by harsh economic realities, was somewhat reluctantly embraced by an elite, who to the outside observer, with hindsight, looked out of their depth and floundering. It was a direct result of poor strategic and financial decisions made, not made or quite frankly fudged, and left the UK state in dire need of hard cash.
Efforts to lure in Middle Eastern oil money - after the battle to control the oilfields had been lost, had taken place prior to the oil crisis from the 1960's onwards. The 1973 oil crisis, when it came, saw what had been described as the most rapid transfer of economic power that the world has ever seen. The West's trade surplus in 1972 was $10 billion, by 1974 the West had a deficit of $48 billion, and the oil producers had a surplus that was estimated to be some $69 billion.
This situation was later aggravated still further in the 1980's when Ronald Reagan (whilst making America great again) and Mrs Thatcher (while not making the UK great) cut higher taxes enabling the mega tick to avoid paying their fare share of taxation (monetarism). This plunged the USA and the UK into a deeper public deficit that they have never got back out of since.
The constant spin that has been put on the relationship between the Brits and the Saudi's (actually the ruling House of Saud) has always been that Saudi Arabia is pro-Western. This was perhaps true to a degree by way of comparison with the Colonel Nasser, Saddam Hussein (although from time to time he was the West's golden boy) and Syria - who were at times openly antagonistic to the West.
The harsh reality was (and is) that Saudi Arabia is pro Saudi first and foremost and the Saudi state's values are directly contradicted by the pluralistic and democratic values of the West. Long before the rapid rewriting the fiscal relationship with the West in 1973/1974 the House of Saud had quietly oenly and consistently funded terrorism and funded a disturbingly fundamentalist interpretation of Islam.
The ruling House of Saud was and still remains engaged in significant internal oppression and has long been well documented as a violator of human rights. Despite this the UK state (as a loyal and purchased friend) supported Saudi Arabia's membership of the UN Human Rights Council - this perhaps gives life to the old adage that what was once bought stays bought.
By mid 1970's under Conservative (under Heath) and Labour (under Wilson) the Saudis had invested an estimated $9.3 billion. Saudi funds were used to subsidise the old nationalised industries to the tune of $800 million. Some $1.4 billion was also borrowed by the UK's nationalised industries. By the time this financially dependent relationship had developed, the Brits had just lost their place as the preeminent regional (if external) power in region to the USA and were fumbling about on the edge of Europe (neither in nor out).
A visible indicator of the drop in the UK states' status may be reflected in the educational choices made by the Arabian and Gulf elite. Once the children of the Arabian and Gulf elite attended public schools in England's Home Counties, now they are more likely to attend expensive private schools around Washington DC's beltway.
For the Brit elite taking second place, even as a financial vassal state, must have stung a bit. This relationship (vassal status by any other definition) explains much, especially the repeated failures of the UK to reduce its dependence on oil and gas imports from the Arabian gulf region - and perhaps the failure to push for a renewable energy based form of energy independence.
It also explains an obvious fact that the UK in relation to foreign policy, had largely and abjectly surrendered effective control of its foreign policy to Saudi Arabia. Saudi interests are effectively Brit interests - it also goes some way to explain the almost complete lack of Brit criticism of some of the more unpleasant public acts of brutality that have been carried out by the Saudi regime over the years.
The Brits for their part have supplied mercenaries to protect the Saudi royal household and supplied troops to fight proxy wars on Saudi Arabia's behalf. In exchange for funds invested the Brits have also helped to arm (to the teeth) one of the worlds more unpleasant regimes in on of the more dangerously unstable regions of the planet - and have happily backed and supported the House of Saud’s not so proxy war with Iran in the Yemen.
Since the 1970’s in exchange for hard cash the Brits have continued to supply training, weapons, etc to a state, whose defence policy (as noted in February 1975) was based on ‘Jihad’. The Brit elite to be frank knew exactly what it was doing when it signed to an effective one way pact with the House of Saud - an over active developer and exporter of Wahhabi extremism.
Short term decisions have long term consequences - Saudi funding of the Taliban trained and educated fighters in Pakistan to fight the Soviet Union, twenty years later the Taliban we doing their best to kill our own soldiers in Afghanistan. Ironically the Taliban used many of the methods they had been aught by the Brits (amongst others) somewhat ironically based upon those methods used against British soldiers in Northern Ireland during the troubles.
Brit policy was a direct result of the alliance with the House of Saud, and it was profitable, between 1985 and 1988 the Brits sold $15 billion dollars worth of weapons to Saudi Arabia. The ready cash burnt some fingers in BAE and other organisations - but Westminster did nothing to curb the corruption - but acted swiftly to prevent any parliamentary investigation into corruption relating to BAE. These sort of deals appear to have bought silence from successive Westminster governments in relation to the House of Saud’s funding of radical islamic causes in the Middle east and around the world - who were opposed to political pluralism, religious tolerance and women rights.
Things got much more complicated after Saddam’s invasion of Kuwait (another non democratic hereditary state) and the subsequent war to liberate Kuwait (funded to a degree by $50 billion dollars of Saudi cash) which saw the deployment of 500,000 mostly 'infidel' troops into Saudi Arabia. With Saddam weakened, but, still left in power in Baghdad, Saudi Arabia itself began to suffer from acts of terrorism. Despite this only in 1994 was Saudi funding of Bin Laden reluctantly curbed along with the removal of his Saudi citizenship.
Before 9/11 the West largely looked the other way in relation the movement of funds for islamic inspired terrorism. The funding continued indirectly via the Pakistani military and private Saudi donors and islamic charities - a 2002 French report noted that $300 million had been moved to Al-Qaidia in recent years.
The Brits effective economic dependence on Saudi money aside from a failure to deal with money laundering (some parts of the City do very well from this activity) has also led to Brit support for some other fairly unsavoury brutal repressive regimes in the Middle East. Ironically it was perhaps the threat of possible European regulation of banking (including money laundering in the City and off-shore dependencies) may in itself have provided the motivation for the campaign that eventually led to the BREXIT vote.