International vs home fees - very confused

Hi there - I'm applying for a doctorate and I've lived in the UK continuously for over 20 years, and also been employed pretty much all that time (including in the English civil service). I was born in Australia but I've had a British Passport since 1996, and I've been married to an English-born man since 2016 (and in fact I was previously married to a Scotsman!). Not only this but my father was born in the UK and lived much of his life here - back in 1996 that meant I qualified for a British passport automatically. Yet the University of Reading is still saying I'm an international student and don't qualify to pay home fees. I'm really confused - can that be right? Have the rules changed? I didn't have to pay international fees for my Master's at the University of Glasgow in 2018-19.... can anyone advise, as the information on Gov.UK and other sites seems contradictory. My UK passport was reissued in 2016 and there doesn't seem to be any information on it to prove that I held a previous UK passport before that.