Hi - I used GiffGaff while I was in UK for 6 weeks over Xmas - and it was great. But there is no option to close account once done - it'll close after 6 months due to inactivity. In the surveys that come out there is no option to say "I was only using GiffGaff as visitor".
I will recommend GiffGaff to friends visiting UK, but an option for visitor style account would be useful I think.
allow over seas members to freeze an account at the cost of a text message for each month
That sounds like a sensible idea. However, it ptobably wouldn't satisfy Ofcom rules.
PAYG numbers have to be recycled if they are not in use and I'm not sure how ofcom defines "use" in this context. But sending a text clearly constitutes using the number even if it's only done as a formality to demonstrate that the number is still being used. Paying the cost of a text, but not actually sending one, doesn't provide proof that the number is being used. So, although they amount to almost the same thing, it seems likely that ofcom would object to an arrangement that was purely financial and didn't involve any actual usage.