Just a thought here -
Not sure where in the UK you are (I assume England), or the program you are considering, or which University, and all that will color the following. We were navigating a somewhat similar hurdle with my daughter. She finished her master's here, and then was planning to go on for a PhD. We finally found a loophole to get "home" fees (as the dependent of an EU citizen who had moved here several months prior to her starting her program - thus meeting the "where would you have been if you had not been in full-time ed here" criteria). But, after doing quite a bit of research, she found out that a PhD in her field from the Uni she was planning to attend is not considered to be equivalent to a USA PhD. The one here is basically research on her topic, and then writing up a dissertation. No additional classes, no teaching experience. She'd come out with a PhD that would be incredibly narrow and no other preparation for a career in academia (which is what she wants).
She'd be quite an expert, but only in that very small area. It would definitely not serve as an appropriate qualification to work in academia in the USA. (Where one attends from three to six years at the PhD level, undertakes teaching, and takes several of those years as coursework, as a rule. The Unis hiring there expect a much broader preparation. And, to my knowledge, none will hire someone to teach who hasn't been trained to teach and actually taught classes.) I am not sure how well a UK PhD would fare in the EU, as far as the requisite career preparation goes; however, with the UK now leaving the EU, and with her not being an EU citizen, it wasn't looking promising. (Had she been an EU citizen I suspect it wouldn't have been as bleak.) She is now hoping to get into a PhD program in the USA. She is comfortable that a PhD from a good school there would serve her anywhere. Unlike here in Scotland, most of the better PhD programs in the States make it financially possible for their admitted students to attend, so that's also in favor of going back to the States for it.
As noted, I don't know your field, or the program you were hoping to attend, or your future plans. But you might want to look carefully at all that. If you intend to work outside the UK (in the EU for instance) you won't be on the same level playing field with other applicants. The EU applicants will have a leg-up on you if you only have a UK degree and are not an EU citizen. The problems my daughter found with working in the USA might also apply to you, if you want to eventually work there.
She did carefully consider if her degree would make her marketable here inside the UK, but with research funding from the EU drying up, and there being very few suitable positions in the first place in her field, she decided it was an unreasonable course of action for her to pursue - she'd potentially end up working in a shop, with that PhD, or otherwise unemployed.
Something to consider. It's a lot of money to invest, and time as well.
(If one of your parents had Irish parents you could apply for Irish citizenship by descent and get the magic EU citizen card, which might be of great use to you later on.)
