I have just been to Bournemouth for my blood test. Asked about the crossover trial. It is starting end of March here, but, they will give you the vaccine/placebo when you are called for your NHS vaccine.
So basically you get the text/letter saying you are now entitled to the vaccine you call them up and they will offer to unblind you or do the crossover trial. So it could still be a bit of a wait for me (in my 40s so not expecting it soon), the good advantage is you will have the 2nd vaccine within the 3 weeks rather than the 10-12 weeks for the nhs second vaccine.