The MA at Morden I think has temporarily closed due to issues with the landlord. I got my betta two or three weeks ago from MA in Ashtead - he's thriving but the other two they had looked quite poor. The MA at Guildford (actually East Horsely, in the garden centre) is my preferred option for livestock - much better range and display tanks - it's just harder for me to get to as I don't drive. If you have transport, I'd give them a ring and see if they have quite a few bettas in, then go along and pick one.
RIP Jimmy
RIP Jimmy