Ahoy and welcome. Looks like you've got a couple of options round there. I'd emphasise that it's just fine to go to airsoft solo, it's super easy to get talking to people, and most folk at most sites will be very happy to show you the ropes.
Welcome Have fun at StrikeForce Gloucester - some really fast, up-close CQB. I played last week there and had forgotten how much fun and how tough the games were. Make sure you take a torch!
Spartan and BlackOps in Bristol are good outdoor sites, also in Gloucester is UCAP Vendetta (in the old prison!) and I think there is one just across the bridge in Wales (Dragon Valley?)