Although Thornbury has closed, they've moved the main site to Lulsgate, so if you're Bristol based it's still not that far. Have a look on their website for directions (Ultimate Airsoft). Alternatives are Dragon Valley (at RAF Caerleon - not to far over the Bridge) or I believe there is one opened near Wickwar (can't remember the name but there is a thread on here about it). Also Airsoft Skirmish, up near Worcester or try First & Only Midlands (The Asylum or The Tunnels) both near Kidderminster, still not too far away.