Process for employer changing my working days

[Babysharkdododont]

I work 3 days per week, set days. A colleague has requested to go part time, with a specific day off, this will only be possible if I change my days. I am genuinely unable to as I rely on Mum for childcare one set day, and have planned nursery etc around that.
Today my boss asked if I could be flexible, I said of course in exceptional circumstances I'd change days, as I always have, but could not be flexible to change every other week etc.
Where do I stand if they now want to change one of my days, how much notice would I be given legally? I looked in the policy but there was no firm answer, and HR were unable to help today. Thanks in advance.

They can’t just change your contract by giving notice. They’d need to seek your consent, which you can refuse. To force it through they’d need to consult you and jump through some hoops which would involve them having a good business reason for the change. To accommodate someone else’s preferred working hours isn’t a good business reason!

Flowery I could kiss you! Thanks so much, I feel hugely reassured. It has been a tricky time and I feel like I've finay sorted childcare, work life balance etc, the prospect of it being taken away was awful.

I know where i work they can actually give us 13 weeks notice of change of shift/day patterns!

Fluffy if there is contractual provision for that variation, and it’s a contract clause/policy which is reasonable for the job you are doing and the variation provision is “live” and genuinely exercised reasonably regularly, that’s fine. But I’m assuming that isn’t the case for the OP as she would have said so, her days are set and there is no policy or anything around regular variance, because she has checked.

I'm writing this as a completely untrained person, but why should her request for a certain day off take precedence over your current days off??

To be fair to the OP’s employer, it doesn’t sound like they are necessarily saying that. They’ve asked the question, which is perfectly fine to do. It may have been that the OP could have swapped days, in which case they could have agreed the other person’s request.

Thanks for the thoughts. I work for the NHS, so the set days are always "subject to the needs of the service" which is fine of course, I'm happy to swap to accommodate patient need, and have done.
The trouble is the colleague who has asked for "my" day off is the office Princess and does a great job of sucking up to the Seniors, so I'm worried they'll accommodate her over me.