I have zero knowledge about quarantine rules today. But... These things would put my rellies off:
My household not fully vaccinated.
They will assume most people they encounter aren't yet vaccinated.
I'm not sure how they are about crowds; I'll let you know on US Mother's Day which is the next big event I expect the relatives to gather -- if my parents go along to that. I doubt very much they will.
They only want to come to UK in warm weather -- early May is the very earliest they would consider. They think of every month as likely to be freezing. About 15 October is the end of the conceivable time window, and 15 September would be better.
They want to come on a cruise ship, they hate the jetlag. Are cruise ships running from NYC to Southampton?
Dad is being investigated for some kind of neck surgery. I have doubts it's required but heyho.
That's without me not having a clue what the current quarantine rules are.
If the current rules are that they would be allowed to quarantine at my home for 10-14d my home isn't big enough & they wouldn't want to pay lots ££ to be stuck in a hotel. oh, and they get bored. Even when they come over to visit my town, they typically only see us for 2-3 hours/day. Not really sure what they do rest of the time enjoy their space? Read books? Old people need a lot of time to do anything. My dad often catches a poker game & step-mum goes walking or swimming.
My TV isn't big enough for them, either.
As for me going there I'm not fully vaccinated, they can't house me + all DC, someone might insist I quarantine quarantine would screw with my mental health. I would like to visit but I'm very unhappy about any isolation periods. Can't make myself deliberately endure isolation for non-essential trip. The high cost & bad jetlag are awful enough.