May I summarise:
You are about to return from mat leave
You were p/t before mat leave (3 days) and now a formal job share partner has been appointed (2 days)
You work in a team, comprising 4 full-time equivalent posts and 5 people.
It's an NHS Trust and your team is responsible for patients receiving a particular type of care (not sure if any of you are patient facing or how much team members are at their desks)
You manage the other team members.
Your role was not covered when you were on maternity.
The four/five staff have previously been split between two offices and now have only one.
Things I am not clear about:
How many of you also work hybrid?
Whether there is bookable space?
If you all deal with patients why you need to work confidentially and why there isn't a culture of respecting confidentiality?
You talk of managing the team, are you on a higher grade and who managed them when you were away?
On the basis of the information I think has been provided, there is no obligation to provide you with a dedicated desk, office, etc, unless it is written into your contract. Your colleagues have also moved office and are participating in this arrangement so it is not discriminatory - when you are all back and there are 2 desks between 4/5 you will all be hot decking.
You are not returning on different terms and conditions therefore there is no sex discrimination.
More broadly:
If your work could be covered without replacing you when you were on mat leave it is an indication your unit/service is overstaffed. Rather shocking if all we hear about lack of resources and workloads in the NHS is true. If this is so, then it is reasonable there should be a business case to review the service.
Overall @Proseccoprincess33 you sound entitled and inflexible and possibly difficult to manage. I think you should go with the flow, be collaborative, and be mindful that at work it is not your desk or your office, it is the organisation's and they can allocate as they see fit.
If there is a business case in the offing, and it sounds as though there should be, you are doing yourself no favours. A business case is likely to be implemented in accordance with a management of change procedure and any risks will be dotted off.
I presume you are a nurse? If so, there will always be work for you and always an income so it is difficult to entirely see why there would be a problem if there had to be a redeployment.
If there is redeployment the management of change procedure may indicate selection criteria: attendance, performance, appraisal record, etc. Only you know those details.
I'd be open minded and exercise some flexibility if I were you.
I would support you over the meeting however but would like to know who requested it. If it is a formal meeting you should have been given the reason for it and given the opportunity to be accompanied. It is more likely to be informal as were it formal, HR should have advised. It sounds as though your manager feels they need support at the meeting and there could be a myriad reasons why.
Reading between the lines I suspect you may not be the easiest employee to manage. It sounds like your manager has not sought advice from HR but then again NHS HR is a unique beast.
Senior HR by the way.