Modern manufacturing has improved such that mixer taps are available that can handle a mismatch in water pressure between the two sides, but that's relatively recent. Historically, you couldn't do it, so we had two taps.
But crucially, this means many British homes have a 500l tank of clean water in the roof that with a little care is available for use in the event of an emergency.
I've even seen talk that the reason this was the norm is cos cold war resilience.
3/n
@quixoticgeek with the move to combi boilers and now heat pumps there's been a move away from cold storage tanks. Most places I've lived haven't had one, though some have a hot water tank. Current hot water tank requires incoming water so turning off at the mains has limited use until the pressure drops.