They made my return to teaching so difficult, I had to resign

I was a teacher with a permanent full time job when I went on maternity leave in February 2015. In September 2015 I wrote my official request to go part time on my return, offering some flexibility – I didn’t mind working 2, 2.5 or 3 days and I didn’t mind which year group. I did ask for the days to be consecutive so as to make childcare simpler.
The reply was that I could work 3 days but only if it were spread across the whole week: I’d work every morning and all day on a Wednesday. I queried this and was told they couldn’t afford to offer anything else. I was baffled as if I had the right to return to my full time post, how would they have afforded that? With a heavy heart I had to refuse the offer and officially resign, having been told verbally by the Head that we could come to some arrangement whereby I either didn’t pay back my maternity pay or paid back only some of it, under the circumstances.
What actually happened was that I am having to pay back the full £3k, about £250/month, as well as not having had any KIT days due to being so upfront and organised in the first place! I try not to think about it too much as it makes my blood boil.