Yes, if you use an UG filter then the bacteria's in the gravel, so you mustn't change it all at once if you're leaving fish in there while you do it. Maybe change it bit by bit and divide it with a gravel tidy in the meantime so it doesn't get mixed up? - I suppose it wouldn't look brilliant while you were doing it but it wouldn't be for long.
I go to the Hollybush too, and I've also started to use Erdington Aquatics, just off Spaghetti Junction, which is v good.