Swinging Heaven is still the most popular UK-only swingers website although its traffic has fallen by more half over the past year because they made the switch from being a free to site to a site with quite an expensive subscription. If you were lucky enough to have an account with them before they made the switch then you can use the site for free but all new users have to pay.
I'm not very sure why they made the change, normally this sort of thing happens after the site is bought by a company that has no idea how swingers sites work or maybe the owners just decided they wanted to make some cash. Either way the loss of Swinging Heaven is quite sad and I'm sure it will lose its number one position in 2007.
At Fab Swingers there's another article on Swinging Heaven.