IMO two of the UK's best companies are 4FW and CSF.
4 Front Wrestling supposedly run Europes only full-time wrestling school, where I trained for a while, and it's really good. Very good and efficient training. But I wasn't there long enough to get on any shows. I've been to some of their shows, and they're very 'high standard'. 97% of the wrestlers are muscular and ripped, and they're all very technical in the ring. 4FW also has a very professional looking website.
http://www.4fw-online.com/index.php
Combat Sports Federation is the company Dave Youell works for, and it's the company I started my training with way back in 2003. Again, I didn't get on any shows because training stopped where I was living, and was too far for me, and I had other things going on at the time. But I have been to many of their shows, and they're great. Really entertaining, with some awesome talent, and high production values.
http://www.csfwrestling.co.uk/index.htm
There was a great 'boom period' back around 2003/2004 where there were loads of wrestling companies around the UK. There were at least four or five within a one hour radius of me.