Hi, As my best mate is a Londoner, and I'm a vintage sports car nut, I've been to London probably 20 times in the last 12 years, and I've never found a decent model shop in the city. Even the vaunted Harrods has virtually nothing, unless you want to spend $20,000 on a model Ferrari. Rents are too high. There is St. Martins Accessories, just up the street from Trafalger Square. I can't remember the name of the street, but if you head up between St Martins and the National Academy about three or four blocks almost in the direction of Covent Garden you'll find it on the left side. It's just a hole in the wall, very small, and sells mostly diecast and such, but some really cool car models. The last time I talked with Ralph, the owner (who, btw, races vintage Mustangs), he was thinking of moving out of town as well because of the high lease rates and taxes he was paying. As a bonus, another block or two will take you to a mews which houses one of the better used book stores in town.
While you're at the Maritime Museum you'll want to allow hours to go through the Victory. It's astounding! The other ships are interesting, but the Victory is incredible. Makes me want to build my Heller's copy each time I see her. There are several gift shops there which sell a few models and several books I've never seen elsewhere.
Hope this helps, though probably not.
Bob