Pingu - I did think there was going to be very, very limited swimming prior to racing. I think one of my local lakes will open about 3 weeks prior (if that). There are a couple of others (Cromhall and the Bradley Stoke lake) who will let you swim sooner.
Raf - that said, I'd say as long as you can get a couple of OW swims in beforehand to remember how the wetsuit feels, there's no huge necessity. Get a couple of swims in out there if required. You gotta come, the heat of the race will suit you 
I'm looking at Monarch out of Gatwick Weds to Mon or Thomson out of Bristol Sun to Sun. I'm currently weighing up the pros and cons.
Pingu - there's flights from Cardiff but they are also Sun to Sun.