Crewing a boat
If you love sailing, or just have an unquenchable taste for adventure and new experiences, you can sign on to help crew a boat just about anywhere in the world, including the U.K.
Of course, this means work, so it's not totally "free" in that sense, but what an opportunity. Resources like the Float Plan (a.k.a. The Cruisers' Companion; see below) helps you find skippers seeking crews and mates to serve on board or just join in a trip for companionship.
(The site is also used by folks looking for others to sail with them as equals, not employees, and therefore share the expense.)
Usually we're talking small sailing vessels, sometimes motorboats or even yachts. You may sign on as a cook, mate, swabbie, or all-around helper doing all the odd jobs. Sailing or boating experience is sometimes required, often not.
But don't worry; if it only took us a day to teach a group of 14 year old Boy Scouts how to sail in a bay off Miami, you can learn it, too. Sometimes you even get a small stipend; usually you just get free room (er, berth) and board. Occasionally, there is a small fee to cover your meals—but, hey: it's a chance to sail on the cheap.
'Course, since you're not the captain, you go where the boat owner wants to go. But if the actual itinerary isn't all that important to you and you just to travel and see the world, this can be quite an adventure.