Strange...I've just tried on my local development site and it's allowing me to make EUR live and the default. Once it's the default then I can uncheck the "live" box on GBP.I did have to restart the site/app pool for the Euro symbols to display rather than £'s.
I guess you could try "brute force" - and update the database directly..
update[YOURDBNAMEHERE].[dbo].[tblKartrisCurrencies]
set CUR_live = 0
where CUR_ISOCode = 'GBP'