I think the whole thing, like so bloody much when it comes to healthcare, is a bit of a lottery.
I've just spoken to my GP, who was lovely, and pretty much offered me the Pfizer as soon as I said how long-lasting my side effects were. I barely had to mention the blood clot issues.
She said they'd had quite a few cases like mine and they'd mixed vaccines, and she also seemed to be up to date with the thinking that mixing gives a better immune response.
I was expecting a battle and I'm so sorry for everyone in this thread who hasn't had such a favourable response.
Because most of all, she listened and said she totally understood why I wouldn't want to risk the AZ twice, rather than insinuating I was a rabid, selfish anti-vaxxer.
If there's anyone further behind my timescale, I'd definitely advise making sure you log your side effects; I've had a call to 111 for the headaches and a GP/hospital visit for the palpitations, which left a paper trail for mine.