The UK embassy advises that you need the cert within 72 hours of flying..so that could mean you can have the test an hour before you fly,as long as you get the cert at.the same time..not that I'm not recommending you do that though..
you legally allowed to travel anywhere..no one can stop you.earlier in the year the UK did have a form of not being allowed to travel to any country for holiday purposes..but that is all done away with now...
Everything is normal to go to.Thailand..my suggestion is take out Thai insurance and fly with either Emirates Qatar or Singapore..the fun and games start when you head back to the UK..
First person that I have heard being stopped going.and it's just bad luck..in a few weeks Scnapps and Co.will do away with it and will be allowed to travel..