I did mean 15.5 DAYS annual leave a year, not hours!
Sorry!!
Surely it’s crucial that we know how many hours a full time working week is in OP’s workplace? Then you calculate what proportion 25 hours a week is of full time, then you know what proportion of 23 days a year she is entitled to.
We don’t know if she works the standard hours each day (and therefore we can work out what the full time hours are) - I assume not, as someone else has worked out that if she did, then full time would be around 31 hours a week, which seems low.
To give a really simple example, if full time is 50 hours a week, with 30 days annual holiday allowance, and the OP works 25 hours a week, she works half of full time and therefore gets half the annual leave entitlement. So 15 days annual leave.
My calculation was working that logic, but with the figures the OP has given, plus assuming full time is 37 hours (which it might not be).