Keep to the left
Stay in the slow lane (the leftmost one) on a U.K. highway unless you are actively passing another car
On dual carriageways (multi-lane divided highways), do not just cruise along in the rightmost, high-speed lane.
In Europe, the fast lane truly is only for passing (or for cars with massive engines that spend their entire trips passing other cars).
In fact, passing other cars has been raised to a fate-tempting art form in Europe, where it is no unusual for someone to pass you at high speed on a blind curve (once, memorably, this happened to me inside a tunnel in Italy).
Please, please, please do not try this yourself.
If someone comes barreling up from behind and flashes their lights at you, it's a signal for you to slow down and drive more on the shoulder so that they can pass you more easily (two-lane roads here routinely become three cars wide).