It should not be too hard to get from the Airport to the site, The airport I think you would need is Gatwick, its south of London (I'd need to check its the best one) or Heathrow.
The main Motorway around London (freeway) is the M25, it circles London, you would need to the the M4 West bound for and the tribemeet venue is not far from the M4.
I think its maybe 2-3 hours max from London to Cwmcarn.
As with anywhere, you can pick up a car from the airport. As for accommodation, you can camp on site, or take the B&B or Hotel option, I'm currently working out where the local stuff is. I have a trip planned over Xmas to find out more about the local area. I have some friends down there.
Sounds good, I have only been there a couple of times myself and it may even give me the courage to try the "bigger" run! I have also only been there with cold weather, so June could be a bit warmer. Def fingers crossed.