Without the requested information, I can only provide you with the following general response, which will hopefully still answer your query.
The key will be what your contract says and whether there is anything in there which states that the employer can ask you to work weekends or any other similar wording which can include this, such as ‘reasonable overtime’. It is all about the contents of that contract and the employer’s ability to dictate your working days or to request that you work extra hours as and when required by them.
In the event that they are not allowed to do this, what they are asking of you will be an unreasonable request and it is something you can challenge them over, such as by raising a grievance to start with. Your next steps will very much depend on what path the employer talks, such as whether they discipline you and dismiss you, or just discipline you with a warning and make your life difficult.
The former will allow you to consider an unfair dismissal claim, whereas the latter could prompt you to resign and claim constructive dismissal.
Either way, you can make a claim int eh Employment Tribunal if needed, but if it gets to that It is also worth mentioning that there is a possible alternative solution to this, which could avoid the need for legal action. That is where the employer is approached on a 'without prejudice' basis (i.e. off the record and with protection against these discussions being brought up in future legal proceedings) to try and discuss the possibility of leaving under a settlement agreement. This can be done by asking for a meeting, or it can be done in writing, via letter or email. Under a settlement agreement the employee gets compensated for leaving the company with no fuss and in return promises not to make any claims against the employer in the future. It is essentially a clean break, where both parties move on without the need for going to the Employment Tribunal. However, it is an entirely voluntary process and the employer does not have to participate in such negotiations or agree to anything. There is nothing to lose by approaching this subject with the employer and testing the waters on this possibility - the worst outcome is they say no, whereas if successful it can mean being allowed to leave in accordance with any pre-agreed terms, such as with compensation and an agreed reference.