Good luck trying to get a reply from the Premier Inn. They keep their prices low by limiting personal contact.
In terms of size, the ones I've been in were closer to a standard size, although I never measured it. Actually if you look at their websites, they mostly list the "standard/double" bed. I don't think I've ever seen a king size bed in a PI, but I've also never asked for one. I would imagine they have both sizes, so I would ask for one if that is a concern when you book.
FWIW, their beds are very comfortable!
ETA: I just looked at their website where they sell their mattresses, and the size of the double is equal to an American double. But that doesn't mean you can't book a larger size. https://www.premierinnbed.co.uk/premier-inn-mattress.html