They're doing it for both Oxford/AstraZeneca and Pfizer, which are the two approved vaccines in the UK at the moment.

Moderna is also approved, although there doesn't seem to be much supply. Novavax should also be approved within the next few weeks.

There’s not really much data on what the most effective interval is, though I think some data might come out of Israel and the NHS will probably try to conduct some randomised controlled trials. The choices made in the phase 3 clinical trials were driven by trying to pick a sufficiently large dose that the trial would succeed and as short an interval as reasonable to make the trials take less time. For other vaccines, larger intervals have been more effective. Given how unwilling politicians were to speed up rollout, I think aiming for maximum efficacy with minimum trial latency was the best decision the drug companies could have made.