British Go Association rules
As of October 2007, the British Go Association uses AGA Rules. The exact details are published here. Of note is the BGA interpretation of the AGA rules.
Earlier Rules (pre-2008)
The default rules used at British Go Association events are Japanese Rules except that Triple Ko is a draw, not a no result. See http://www.britgo.org/rules/approved.html
RobertJasiek: Wasn't there more? What about no Both Players Lose rule?
LithiumTwo: There is a Both Players Lose rule, but as it is mutally exclusive to another rule, the BGA maintains the right to decide which one to rule in favour of.