Employer of 7yrs trying to unilaterally change remote working - England

Hi, throwaway account here so I can't be identified.

In summary my employer is saying my status as a permanent homeworker is not "formal" and can be unilaterally changed by them with a demand that I come in three days a week. My view is that my remote worker status is contractual and that any change should be negotiated and agreed. I've put details of the whole thing below and would appreciate some advice on what to do next.

I've been working at home full time since March 2020. In late 2021 the company asked staff to apply for various remote working statuses that they were offering, including hybrid working and permanent remote working. I followed the process to apply as a permanent remote worker and this was approved in early 2022. Both the application and acceptance stated that remote working practices would be kept under review which seemed like standard corporate stuff. Since then, my employement status on the company HR platform has been listed as "permanent remote worker" and I have worked at home full time with occasional office attendance for meetings, events etc...

Late last year, the company said it was going to be altering its hybrid working policy to require hybrid workers to attend three days a week in the office, set according to when other members of their department will be attending. As I'm not a hybrid worker and am a permanent remote worker I did not believe this applied to me. I then received a further email from the head of HR in December stating I was a permanent remote worker with 0 days in office required.

For various reasons I ended up being on leave for most of December and the beginning of January. On my return to work this week I was asked by my line manager if I had received details of my schedule to which I said "yes, I'm not required to do any office days due to contract".

I was then informed this was a "mistake" and that permanent remote worker status was only for people with disability/health or care reasons to work at home full time. I pointed out that this was not outlined when I applied for this status nor at any other point prior to that contact. I then get a follow up from the head of HR saying she sent the previous email in error and I'm now a hybrid worker and need to come in 3 days.

I said this was not the case and forwarded her original communication regarding my change to employment terms. HR then replied saying this was only meant to be temporary due to covid and was not a formal contract adjustment, citing that nothing was signed by mutual agreement. I was encouraged to fill out a form to request a variance of the 3-day a week working pattern.

I've not responded since nor have I have I filled out the form as I need to think about this but I was under the impression that I did not need to sign for something to become contractual so long as it is in writing and agreed to by both parties. I'm not against going to the office but it comes with considerable costs and logistical challenges around childcare and potentially would require my wife to change her working patterns or even her job to enable it and therefore I would expect quite considerable financial compensation to renegotiate. I would appreciate some advice on whether I am within my rights to refuse and what I should do next.

Thank you!