My friends in the 50 - 54 year old priority group 9 got wind of the online booking being opened up before the official announcement was made (other friends knew and had told them). They said they seemed to enter records by area and by age (so if your area was open, they started to add records by age from oldest first) onto the system to book.
The person who told them had been able to book mid afternoon (they lived in a different authority). They tried themselves at 6pm they couldn't do it, but when they tried again at 10pm they could. It was done on a Monday (the 15th - I'm just checked my phone messages for the day).
From what was said on MN, not all areas were rolled out at the same time - some were added later than others.
It had been rumoured in the press on the Sunday night and then it was seemingly confirmed in the press on the Monday night front page releases and then officially confirmed on the Tuesday.
Hopefully that gives a possible template for when/how this will happen.
I suspect that they will open bookings but they may not be many available in April in many areas. But even giving people a date in May I think will help to steady a lot of 40 something nerves.
Group 10 (40 - 49 year olds - its the full ten years not 5 as previously) is theorectically 5.3million in size (although lots of people will be already vaccinated because of their job or for health reasons - it looks like its closer to 4.9million left to do from the Omnicalculator data). Group 8 (55 - 59 year olds) was 2 million and Group 9 (50 - 54 years olds) was 2.4 million.
They were still managing to do 100,000 first doses a day at the end of last week, even with second doses prioritised. It might be a slower roll out for the 40 - 49 age group at least initially. I think the over 50s have taken about 6 weeks to complete (with some areas doing it quicker and cracking on with the over 40s as they seem to have the doses).
So possible all over 40s by end of May seems a viable possibility if we have more deliveries in May by my fag packet calculations.