Quite interesting, this. I'm term time + 5 days, and I love that, as it means I get some time with my children, and I'm not about to give it up - but if I'm being honest, I could do with more time here in the holidays, for all of the reasons stated above. (Don't tell my bursar!)
If they are really looking to cut you down, a left field suggestion might be that you ask to do the PAT testing - I think you just need a 1 day course and a piece of equipment for a couple of £100s. That might let you keep your full year contract, but give the school the ability to get that disruptive job done during the holidays, with some flexibility in when you do it, and when you work on other things being an advantage to the business manager also. Just an idea in case it is useful.
Good luck!