If you arrive in London via Heathrow, the Sheraton Skyline houses the Collector's Aircraft Models (CAM) shop, which has a huge selection. However, be warned: prices are outrageous at the 2-to-1 exchange rate. But if you just want to look, I'd recommend it.
I'm sorry I don't know of any good diecast stores in the other cities, even though I 've been to all many times (in Paris, there are a couple of military book stores that also have some diecast, but limited selection. There are however several very large plastic-kit stores, if you are interested). I guess I've gotten used to getting my stuff on-line back in the States, what with the prices here being so high.
FVD
Oh, and datacroft: that's only three capital cities...
