Cohorts 5 and 6 are going to be eligible really soon, given the government has a target to offer everyone in cohorts 1-4 the jab by mid Feb. And as this thread has shown, some GPs are already doing some people in cohorts 5 and 6...
The booking link originally said you needed a letter to stop millions of people crashing the system at once, not being able to get appointments, and because the vaccination centres and pharmacy sites don’t cover the entire county. So the NHS didn’t want to say anyone can book, only to get loads of complaints from people in deepest Norfolk, or Cornwall, or other rural areas, complaining they’re miles from one of the sites bootable on the link (even though they’ll be a GP site much nearer to them).
However, now a good number of the over 70s have been done, and there is still really good appointment availability at the vaccination centres, I think we are very likely to see that message (about not booing until you’ve got a letter) changing soon, given the government wants as many over 70s and clinically extremely vulnerable to have been jabbed by mid Feb.