How to politely decline a holiday?

You should align with your bf on this and deliver the same message. A polite rejection only needs to be that you are making other plans for this year and this should be accepted. If secretly he would quite like to go (it’s always easier with your own parents than someone else’s) then you need to find a compromise that works for both of you

My boyfriends parents really want to go away with us. They’ve booked a holiday to Cyprus in June and invited us but I really don’t want to go. My boyfriend isn’t fussed either way( I don’t think he’s been in holiday with his mum since haas 11 anyway, he’s now 23).
I love his parents but they’re renting an apartment and I just think it’s gonna be too on top of each other. My boyfriend & I are well travelled and have only really gone away as a couple where we are free to do what we want.
In all honestly I don’t really like Cyprus, I’ve been before and it’s way too crowded. They said we could just come for a few days but I would rather save my holiday days somewhere else!
I feel very stuck up about it, but it’s not my thing at all. My boyfriends mum keeps asking ME and not my boyfriend if we are coming but I want my boyfriend to decide and tell her cos I don’t think it’s my job. My boyfriend is completely impartial to it - he said it’s a ‘free’ holiday cos they’ll pay for everything minus flights but I would rather pay and go somewhere nicer.
He says that’s absolutely fine but his mum won’t stop asking and I feel awkward about it

Do you work? If I was in this situation I would opt for a little white lie and say you couldn't get the time off work, June is peak holiday time and at least where I work you have to get your bookings in asap or you miss out.