I wouldn't say Dragonball Z is unpopular in the UK by any means, however it is more of a case that, it isn't as accessible for us in the UK. Manga wise, we can get a hold of it, anime wise, the only way that i know of is by importing from the US, since it has never had an official release here, Manga UK i believe had something to say on the matter there.
- Oh wait, wasn't there two of the movies released here? Regardless.
I think DBZ certainly had a role to play in some respects with fans who came to discover anime through watching it on channels like CNX, alongside Cowboy Bebop, Outlaw Star and Gundam Wing, but again, the UK hasn't had anymore exposure to the series bar through the TV or the games. You'd need to find someone with Viz details for manga sales