I'm UK based. Just got an offer for a job where my hours would be flexible and "not fixed", but with a guaranteed annual pay. The job is in a startup - when applying I was well aware that it wouldn't be a clock-in/clock-out kinda affair, where sometimes I would need to put in more work and sometimes where could be quieter times. I've met my employeers and I trust they have good intentions but still I feel slightly uneasy at the possibility of essentially signing a waiver allowing them to request I work fantastically long hours.
How can I protect myself from potential exploitation? Would it be ok to ask to have it changed to set a cap for hours worked over a longer period of time, e.g. a month? I'm not a fan of settting a rigid weekly limit as I know there are likely to be crunchtimes and I'm ok with that. It's more about ensuring that long-hour work doesn't become the norm.