You can get a day ferry over to Cowes Harbor on the Isle of Wight. That's probably the 'capitol' of sailing for the UK. The town of Cowes is charming, but Falmouth on the other end is really quaint. I've sailed several times out of a town at the lower end of the Southampton estuary, called The Hamble. Nice sailing harbor and a really quaint, old town; you'd almost expect to see the proverbial old peg legged, eye patched, and hook handed sailor clomping along the cobble lanes. The Key pub is where some of the yacht sailors hang out and it's good for food, too.
Near to the center of London you can visit Greenwich Observatory with the museum and see Sir Francis Chichester's yacht. It's worth seeing the exhibit of the first chronographs used to discern longitude.
Sure wish I was heading over this year. Hoist a pint for me, chap!