It seems that the original plan was to only offer an alternative to AZ for the under 40s. I'd have no problems if they changed it to offering a genuine choice of vaccine for the under 40s. It would mean that in places like Blackburn people would be able to get AZ on the vaccine busses and that would be a fairly rational decision there. But to remove choice to under 40s, particularly after offering alternatives to the least risky portion of that age group (38-40) is just going to increase the number of people remaining unvaccinated.
Also, I can't tell if this is a rational thought or not... But I'd be very hesitant to accept AZ as we near the peak of a 3rd wave. If hospitals were anywhere near overwhelmed, I'd be highly worried that a persistent headache could get dismissed by overworked medical staff. We know survival from VITT is better with early diagnosis, that isn't possible if medical care becomes harder to access.