Hi there
I hope you have a great trip to the UK.
As the others have hinted... Central London is a bit of wasteland when it comes to model shops... there used to be plenty, but I've watched them close one-by-one over the last 10 or so years.
Modelzone is really about the only one left. As Jakkels says, it's very mainstream, but it usually has a Sale on... so you might pick up a few bargains - particularly Revell stuff. :-)
Hannants is the obvious London target, but you could also check out:
Comet Miniatures
46-48 Lavendar Hill
London SW11 5RH
Tel: 020 7228 3702
http://www.comet-miniatures.co.uk Comet is a bit unusual. Their main shop is all Sci-Fi models, but next-door has an extensive range of military stuff and a lot of "loft clearances" - so there are some real collector's items on the shelves... well worth checking.
I haven't been to the new 4D shop yet, but their old ones (they've moved several times) were great! - plenty of hard to find stuff (materials rather than kits).
All the best
Rowan