If you are planning to have fertility treatment, you need to do some research into success rates with women your age, costs, available treatments etc and then choose a clinic.
If you just want one or both of you to be checked over and to have a few tests, it doesn't matter all that much where you go, although if you are likely to want treatment after your tests, it might be easier to have it all at the same clinic.
Area of London might be relevant, in which case you could try any of the following, all of which can do a semen analysis
The Bridge Clinic near London Bridge CRGH on Great Porland Street The Lister Clinic in Chelsea Create in Wimbledon and St Pauls
You could alternatively try a private GP or I'd be surprised if your husband can't get a semen analysis and results interpretation from one of the many outfits doing tests on Harley Street.