The full rationale for setting the gap between doses at 12 weeks can be found at the link below. Briefly, with both vaccines providing a pretty high level of short term protection against the virus after the initial dose, more vulnerable people will get pretty good protection more quickly than if the original timetable was kept to.

There is still a bottleneck on supply of the vaccines, and the sheer logistics of organising such a huge mass vaccination programme over a short period of time are daunting to say the least. I am not remotely a fan of the governments pretty shambolic response to this crisis in many ways, but these actions on vaccine timetabling do seem to me to be sensible.

https://app.box.com/s/uwwn2dv4o2d0ena726gf4403f3p2acnu