I've just checked and unfortunately GNAS/ArcheryGB is following World Archery's lead and banning the thumb release. I will see what can be done to get the rules changed, but at the moment, no thumb releases allowed in competition. This doesn't mean that you can't shoot with a thumb draw, just that you wouldn't be able to compete in the trad category in a record status shoot.
If it's not record status then it's up to the judge in charge and tournament organiser to decide if you can shoot as trad using that technique. Of course you can always shoot in a record status shoot and not compete, or put yourself down as compound unlimited. (ridiculous I know) Classifications up to Bowmen are club administrated so you can get up to Bowman, but not MB or GMB unless we get the rules changed. Also be aware that most GNAS field shoots are record status.