You just missed the Southampton boat show so yes London in January 2012. Usually they have good deals and if you run around you can compare prices.
Or research on here a lot and buy off ebay or off here. They are all grey and made of rubber and look pretty much the same.
Decide what size (and weight) you want then take it from there.
I'm chuffed with my second hand mint condition Excel.
You will need a motor, fuel tank, lifejackets, ropes, an anchor, smoke flares, oil and loads other bits.
This is a super thread
What kit do you carry on your SIB?