Iconic of this assumption is the mocking of former prime minister Tony Blair as George W. Bush's "poodle" for his riding shotgun on the ill-advised American stagecoach blundering into Iraq in 2003. Blair was in good practice, having served as Bill Clinton's dogsbody in the no less criminal NATO aggression against Serbia over Kosovo in 1999.
On the surface, the UK may seem just one more vassal state on par with Germany, Japan, South Korea, and so many other useless so-called allies. We control their intelligence services, their military commands, their think tanks, and much of their media. We can sink their financial systems and economies at will. Emblematic is German Chancellor Angela Merkel's impotent ire at discovering the Obama administration had listened in on her cell phone, about which she – did precisely nothing. Global hegemony means never having to say you're sorry.
These countries know on which end of the leash they are: the one attached to the collar around their necks. The hand unmistakably is in Washington. These semi-sovereign countries answer to the US with the same servility as member states of the Warsaw Pact once heeded the USSR's Politburo. (Sometimes more. Communist Romania, though then a member of the Warsaw Pact refused to participate in the 1968 invasion of Czechoslovakia or even allow Soviet or other Pact forces to cross its territory. By contrast, during NATO's 1999 assault on Serbia, Bucharest allowed NATO military aircraft access to its airspace, even though not yet a member of that alliance and despite most Romanians' opposition to the campaign.)