Well, the oldest, started in 1970, is All Star Wrestling (Wade Barrett also wrestled in a few of their events). Other promotions are Frontier Wrestling Alliance, Grand Pro Wrestling, International Pro Wrestling: UK, Real Quality Wrestling, Triple X Wrestling, XWA and UKW.
Real Quality Wrestling is probably the easiest to find as it airs on TV throughout Europe. As for the best? I don't know, but, for what it's worth, Triple X Wrestling was voted Best Promotion of 2009 by UKFF.