oftentimes taking advantage of the fact that the US congress refuses to allow certain sales to go through. Here's a pretty comprehensive (albeit slightly outdated) history if you're interested (produced by CAAT - the campaign against the arms trade):
"Saudi Arabia’s dependence on oil is mirrored by the UK’s dependence on Saudi Arabia as its main customer for military products. Neil Cooper describes the UK’s "overwhelming reliance" on Saudi contracts in the early nineties, a reliance which has resulted in a dangerous "condition of dependency" that has repeatedly undermined the independence of our government (Cooper 1997, p149). UK government policy has frequently been dictated by the need to retain the Saudi market, often eroding public accountability and the integrity of government institutions.
The benefits to the UK are far less clear than the ‘Thousands of Jobs Saved’ headlines would have it seem: expert economic analysis has shown that both the billion-pound Al Yamamah deals and huge government investment have paid few real dividends. The fact that the bulk of payment was made in oil under a barter scheme meant that the Saudis were often behind with payments due to variations in oil prices; as Treasury official Robin Fellgatt commented, "an arms sale on credit if someone does not pay up is of no economic benefit, quite the reverse" (Koorey 1995, p51). Economic benefits which are, at best, unclear provide little justification for unethical export policies.
The real profit has most likely been into the pockets of those who orchestrated the deals. The pervasive scandal and rumour surrounding Al Yamamah suggest that the motivation behind supplying the kingdom may have come from those few individuals who directly benefited from ‘commission’ payments. The Dooley court case in the US in 1991, the 1994 Granada TV ‘World In Action’ programme, the allegations concerning Mark Thatcher in 1994, the 1996 Westland memo on the Saudi dissident Mohammed al Mas’ari, the Rolls Royce court case in 1997 and the Jonathan Aitken affair, all support suggestions that, contrary to UK and Saudi law, "commissions are an essential part of the system" (Times, 12.10.94).
Ever since the signing of Al Yamamah I there have been repeated media allegations and many demands by politicians and the public for transparency. However, the government has only launched one investigation into the corruption associated with the deals, an internal inquiry with a narrow remit, the findings of which were suppressed. As Dr Kim Howells, MP for Pontypridd and member of the Public Accounts Committee (PAC) said, the non-publication of the National Audit Office (NAO) report was "most unsatisfactory. If we can’t see the report, and it goes right to the heart of the problem, what does the PAC exist for?" (Observer, 10.5.92). Martin O’Neill, the then Labour Defence Spokesman, pledged at the time that a Labour government would re-open the inquiry, however, the present government has maintained its predecessor’s refusal to release the NAO report (Independent, 12.3.93)."
http://www.caat.org.uk/information/publications/countries/saudi-arabia-intro.php(nb - the Al Yamamah deal was described by the Financial Times as "the biggest sale of anything to anyone by anyone."