Log onto
www.ppjn.com or join the IPA
www.ipapilot.com . They will give you a list of all the air taxi operators in the UK. These companies rarely advertise so you just have to get phoning and writing letters.
I can't recommend air taxi highly enough. I did it for a few years, then got a job with a regional airline on turboprops and now have a job with BA. It's hard flying single pilot air taxi but you learn very quickly. Not only does the experience of constantly having to manage problems on old aircraft prove invaluable but you don't half appreciate how lucky you are when you look back on the experience whilst eating your hot meal in a warm cockpit at 39000'!
Some airlines frown on this type of experience as you can very easily develop bad habits. Just be disciplined when you fly single pilot and don't let your standards drop no matter how tempted you are to ignore VOR's and NDB's in favour of GPS.
Hard work but great fun!