I attended the London show last year and it was fine. London is pretty far from the UK yachting centers, whereas Southhampton is in the heart of those centers. If you like attending boat shows you'll still find the LBS interesting and different from many of the US shows you may have attended. One nice thing, as a non-UK resident you will get VIP treatment at no extra cost. Bring your passport to the show and you can purchase
the VIP-pass (I don't recall
what they really call it), which gives you access to every day of the show (locals pay for every day they attend), plus a nice VIP lounge overlooking the main concourse.
The show is located on the east end of London, far from the center. However, public transportation in London is so much better than any city in the US it's a snap to get to the show from wherever you are staying. Normally just a tube ride then transfer to the Dockside Light Rail system.
There's certainly no reason to avoid the show unless you don't like going to boat shows