I'm club secretary for Kingswood Running Club, and we do run around Langdon Hills. I've done 5ks up there, but wasn't organising.
I don't know much about the other clubs, to be honest. I think Bas AC are more into track than road, Billericay Striders have lots of members, and some very good runners, but I don't know how social they are. Pitsea RC organise races, and they have track sessions at Gloucester Park in the week.
I see lots of Phoenix (I think) runners out in large groups - they seem to set off wearing too much, most of them have jackets tied round their waists!
The websites of the clubs should give you some clues about them and their activities. Try contacting them - they should be happy for you to go along for a couple of runs before committing to joining.
Kingswood is a very small club, very friendly, but not very competitive. When we enter races it's for fun, although one member has won her age group in a couple of races 