This page hosts a link to the current BGA Gliding Frequency Reference Card. You can find it here.
That contains the frequencies of the airfields hosting gliding clubs. In general, the frequency given is the one a cross-country glider (or passing GA aeroplane) would first call if inbound to that airfield. That could be:
A dedicated frequency for those clubs with their own.
One of the shared "Common glider field frequencies", indicated with a shade of green on the colour FRC.
A MATZ frequency for military clubs. In some cases, a weekend frequency is given in the notes for when the MATZ is inactive.
Glider pilots: If you're a glider pilot, it is recommended that, rather than use the link above, you should sign up to the data here. That way, you'll be notified of any changes that happen (and you'll have access to all the other airfield data that I publish).
Power pilots: If you would like to be kept up to date with changes to this file, please sign up for the UK Airfields and Outlandings distribution here. You'll get the occasional email telling you when the gliding data package has changed, with links to it. Whilst as a power pilot you may not be interested in most of that data, you'll be told if there are any changes to the Gliding Frequency Reference Card.