For UK conferences with a free software / UNIX bent, I can't speak too highly of the UKUUG conferences (they have a LISA conference in spring and a Linux-specific one in the summer). They have a very academic look-and-feel, to the point that some speakers still use LaTeX to do their slides; I've found them correspondingly cutting-edge, with a lot of stuff coming out of academia well before the commercial market wakes up to it; and they're extremely cheap by professional conference standards.

I try to go to one every year, and spoke at one of the 2009 conferences (an experience I very much enjoyed).

This year's ones are the Spring LISA conference in March, and the summer Linux conference, which doesn't seem to have any dates yet. If anyone does go, I strongly encourage you to consider giving a talk, if you have detailed knowledge on any relevant subject. It was much less scary than I thought it would be, you get conference registration for free, and the audience are highly receptive to and appreciative of a technical talk (though they can be very hostile to shiny marketing blether, don't do that).