It's my feeling that the MHRA is taking into consideration that we wouldn't have enough doses of Pfizer/Moderna to have two jabs for all the under 60 (or under 65) so in their balance of risk/benefit, they are acknowledging that the risk of dying from covid for people between 30 and 60 would be much higher than having the risks of developing blood clots. Other countries that have decided not to use the AZ vaccine for under 60 (or 55, or 65) have ordered sufficient number of other vaccine doses.
Canada is a good example, they've been using mostly Pfizer and Moderna and have access to few doses of AZ, so for them it was a quick and simple decision to not offer AZ to those under 55. THeir vaccination program has been slower than the UK and they are entering their third phase now, but even if they would still be offering the AZ vaccine to everyone there wouldn't be enough doses to make the vaccination program go faster.
It's a very fine balance isn't it. I wouldn't be surprised that if we can get a hand on more Moderna doses, the age will be changed to under 40 year olds.