Hi, Daisy. Hopefully I can help you here- I live in London and shop frequently with my boyfriend.
The department stores would be a good place to start- Selfridges, House of Fraser and John Lewis are all along Oxford street, and have men's departments. House of Fraser and John Lewis are traditionally very "posh middle-class" and carry several good-quality but reasonably priced brands. Selfridges is huge and has everything from high street concessions (high street is what we call mainstream chain stores here) to very expensive designer labels. There's also Harvey Nichols and Harrods in Knightsbridge, but they can be pricey and I don't actually find the selection that good.
As for the high street stores themselves, we have a lot of what you'll find in the US- H&M, Levi's, Banana Republic, etc. These things tend to be a lot more expensive here, so if I were your husband I'd stay away from those, and focus on the things he can't find in the US!
I'm not sure if your husband ever buys things at Gap, but it's worth remembering that the UK lines are designed by a separate, London-based design team, and are completely different from what you'll find in the US. I rarely shop there when I'm at home in Canada, but like a lot of what they carry here. It might be worth checking it out?
Reiss is a brand that's at the upper end of the high street range, and makes really nice things for both men and women. Their style is very high-end in feel, and the quality is good, too.
My boyfriend's favourite clothing shop by far is COS. Short for Collection of Style, COS is actually H&M's "high-end" line, although the branding is completely separate from H&M. The quality is also much better, and the styles tend to lean toward high-end fashion. Good basic shapes with nice details, that kind of thing. So far they are only in Europe, but I think there might be plans to expand to the US in the next few years. There are two shops in London- one on Regent Street and one in Covent Garden. You have said that your husband recently lost weight- is he very slim now? COS has a range of sizes, but it's particularly good for slim builds. My boyfriend has broad shoulders but wears a 28-30 trouser, and they have the best selection for his shape.
If you give me a better idea of your husband's style, height and build, I can think of some more shops that might work for him.
