There used to be an Apple Expo in London, but a long time ago Apple basically killed it - some of the fatcats in the US thought it didn't make sense to have more than one show for "Europe" (it's all one market after all!

), and standardised on Apple Expo Paris as the big pan-European show.
Of course that didn't work too well because there really isn't the same conference mentality in Europe. An independent company saw an opportunity and organised their own show in London, called MacExpo (didn't have the rights to the Apple Expo monicker). The first one was fairly small but a success, and when it became clear that it would work, even Apple started exhibiting.
So there's MacExpo in London (previously at the Business Design Centre in Islington, but this year it looks like it's going to be in Olympia), and Apple Expo Paris. The MacExpo guys also run them in Germany and the Netherlands.
In my opinion, the MacExpo generally sucks. Boring and small... I think I spent about half an hour on the floor of the one two years back, and didn't even bother going to the one last year.