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 <a href="http://summer2009.ukuug.org/Talks#line-28">spoke at one of the 2009 conferences</a> (an experience I very much enjoyed).

This year's ones are <a href="http://www.ukuug.org/events/spring2011/">the Spring LISA conference in March</a>, and the summer Linux conference, which doesn't seem to have any dates yet.  If anyone does go, I <b>strongly</b> 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).