Separate hot & cold taps: Historically the UK didn't use mains pressure hot water systems. Instead they used boilers that fed a header tank causing the hot water to be at lower pressure than the cold supply. If the two systems were joined, such as happens in the plumbing behind a single outlet, when some mix of hot & cold is selected then the lower pressure cold can feed into the hot pipes.
Even a single outlet in the UK isn't really: Inside the faucet are segregated hot & cold supplies that independently supply the exit point- which is where the two streams finally meet.
Of course a place might have been modernised and gained pressurised hot water & mixing faucets.
Last edited by Tinstaafl; 28th Jan 2010 at 15:06.