Have you submitted a flexible working request? If so what is the reason given in the letter for not accommodating your request?
Not an expert but there was a recent change in the law which strengthens rights for employees https://www.edenscott.com/blog/changes-to-flexible-working-uk#:~:text=Employees%20now%20have%20the%20legal,continuous%20service%20(Personnel%20Today).
In short, yes they can do that as it isn't financially viable for them to do this. Maybe consider 4 full days or going in 5 days but reduced hours? I'm going back for 4 full days
I believe it's managers digression whether you can condense your full time hours into less days because of the numbers of staff needed each day etc. But you should definitely be entitled to the extra annual leave. Whilst I've been on maternity leave we have been awarded a pay rise and I reached my additional day annual leave for 5+ years service in the NHS and I will be given both of those on my return x