Can I be forced to take half a day’s leave for leaving an hour early?

I started with my employer over 15 years ago, working full time Monday to Friday 9-5. I had my son a few years ago and returned to work following maternity leave working Monday to Thursday 9-5. There was an unwritten rule that you could occasionally work your lunch if you required to leave early. In 2015 I was tuped over to a new company under the same contract. Initially the same unwritten rule applied with regard to working lunch. However, this option was removed and instead a form of ‘flexi time’ was introduced (no time was allowed to be built up, however you could start anytime between 8 & 10, but had to work to 4 or 6 etc) My husband occasionally works away from home and on those days I need to do all the pick-ups and drop-offs so flexi does not work for me as I can’t get in any earlier than 9am and I need to be away by 4pm at the latest. I was previously allowed to take an hour’s holiday. I have now been advised that the minimum holiday allowed is half a day. This would mean that over the course of a year instead of 16 hours holiday I would be required to take 56 hours which equates to two weeks holiday. I cannot afford to lose that amount of holidays as then I will struggle for school holidays. Can they keep chopping and changing how this works (even though it’s not in my contract)?