chocbiscuits it depends on a lot of things. What does your contract say about varying working hours/working arrangements? Sometimes there is something about the employer reserving the right to vary these things, with good business reason.
They are saying you have become isolated and need to 'have cups of tea with people' (!). I think the first thing you need to do is ask for more detail about how your work/work of the team is suffering because of you working from home so much. They haven't given you enough information and you are clear that you don't feel isolated so I think at the moment they haven't given you enough of a reason, so ask for that first.
When you put in an application for flexible working, it is considered a permanent change. When you changed, you say that Wednesdays were left 'vague'. Do you have a letter that you were sent confirming that your application was successful and outlining your new working hours. That should be quite clear on what was to happen about Wednesdays. Even if it wasn't, they have happily allowed you to continue working from home on Wednesdays for a significant period of time, so you can consider that as a permanent change to your contract.
Employers can vary terms and conditions of an employee's contract if they consult the employee, explaining and discussing the reasons for the change. Changes should be agreed between both parties otherwise it is a breach of contract.However, you will appreciate that there are occasions where an employer has absolutely no choice but to vary terms and conditions for whatever reason so there has to be a mechanism whereby they can do this legally if they have to without the agreement of the employee.
This is explained below by Acas -
'If, after negotiation, agreement on a variation of contract has proved to be impossible, an employer can ? having followed the statutory dispute resolution procedures, where they apply, and observed any relevant procedural agreements - terminate the original contract, with proper notice, and offer a new contract to the employee, including the revised terms. There will be no breach of contract as a result of taking such action. If the employee accepts the new contract, continuity is preserved'.
It would be considered a dismissal if they did this, so you would be able to claim unfair dismissal if the way they did it or the reasons given for dismissal were not fair. Bottom line is it is unlikely to end up going that far. You need to check the original contract you signed when joining to see whether your employer has put in the right to vary terms and conditions/working location when necessary, and ask for further clarification about the business reasons why your existing working pattern needs to change.
This is a change to your terms and conditions even though it was not written down, so they do need to follow a correct process to do it, consult and discuss reasons with you etc, so your first step should be to demand that.
If you carrying on working under the new terms and conditions without objection, you will be deemed to have accepted them. Alternatives are to carrying on working but give written notice that you are doing it under protest and consider it a breach of contract and will be seeking appropriate damages.
I would strongly advise speaking to ACAS or a solicitor before doing something like that - if you were to bring a unfair or constructive dismissal claim the tribunal would have to be convinced that the change to your terms and conditions was substantially different and could be considered a new contract rather than a variation to the old one - I'm not convinced a tribunal would see a request to work from the office on Wednesdays rather than at home as substantial enough.
If it wouldn't be a substantial change in the eyes of a tribunal, you could instead claim breach of contract in a civil court.
All this is very dramatic though, and I think you need to check your original terms and conditions re contract variation first and foremost, and insist on proper consultation discussion and information from your employer. Speak to Acas as well to make sure you are fully informed, and if you don't manage to do all this before Wednesday, send an email saying that you will come in as requested but you are giving them notice that you are doing this under protest and do not accept these new terms while you take advice about your rights in this area.