Have you submitted a flexible working request? If so what is the reason given in the letter for not accommodating your request?
They don’t have to accept your flexible working request because it sounds like if you do 3 full days they won’t be able to employ someone to cover the other 2 days. So what they’re saying is if you work 18.5 they can employ someone to job share the rest of the hours with you. If you do 18.5 you could work 8-3 Monday and Tuesday for example and then 11-5 or 9-3 on the Wednesday so really you are still doing 3 days x
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