PC Towbar Services (pctowbarservices.co.uk) - 01752 563311 or 07740 783975. They are a mobile outfit and will come to home/work. I've used them twice (on the recommendation of a BMW main dealership) and they have been first class. Very competitive on price and won't fit 'dubious' wiring kits. Dedicated looms are the way to go or your tow vehicle electrics nowadays get confused and expensively unwell. Owner/manager is, I seem to remember, a chap called Gary who came out and did the second installation for me when one of his fitters had to cry-off at short notice.
Based in Plymouth, but will come up this way. Used him a couple of times previously, and he's fitting one again for us tomorrow afternoon. Must resist temptation to change car so frequently!
I used Brittania on my last car and received a good tip beforehand. If you have one, get your local friendly garage to book it in for you. They're 30-40% cheaper to trade customers than if you ring as a private individual. This turned out accurate in my case.