I don't believe the UK has any rules against multiple citizenships [1] -- speaking as a triple national with UK citizenship, if I may ask, what happened there?
Having left I no longer paid much attention - I have no idea what the actual rules/requirements are for EU citizens with an additional non-EU citizenship to naturalise into the UK. And honestly I was so destroyed by the referendum anyway that I doubt anything would've kept me in the UK. Crazy sad.
Otherwise, pardon me, I'd be shocked the UK keeps on growing inwardly and isolated from the rest. I just hope that they embrace the rest of the world as their leaders say. The US would love to have a closer relationship and I suppose other non-european countries too.