Hiring a captain to buy a yacht

Best way is word of mouth referral. Ask around boatyards, marinas, etc. Good captains looking for work are known by other captains, my dockmasters, by boatyard managers and workers. Many yards also have restaurants on site and they're typically filled with industry pros who might know.

Welcome Lloyd, and a good plan. A captain can save you a bunch at purchase and weed out some trash that happens to look good.

You're actually in a pretty good place right here to start. Where do you intend to home port the boat? Once you state that I suspect you'll hear from a few here. Otherwise Olderboater's suggestions are on the mark. PLEASE go beyond asking only if the captain is licensed. Ask how long they've been licensed and about their experience. I'd recommend not hiring anyone without less than 5 years of professional experience, as many dreamers get a license based on having a boat registered in their name. When they find out how hard it is to find work as a captain they're gone when it comes time for their first renewal (5 years for a U.S. license). You should be looking for experience on similar vessels and in the area you intend to cruise. Good luck.

There you go. Let's see if anyone knows a captain they'd recommend for those areas, and I guess London as well. Good luck, and also get word out to marinas and boatyards.
Not sure if you can, but you might want to merge this thread with:
http://www.yachtforums.com/threads/yacht-captain-wanted.24226/

All those who followed that thread will be notified of your post, plus it'll be easier to find with the search feature.