You are under no obligation to agree to what is a fundamental change in your terms and conditions of employment. If you agree, you are not entitled to any compensation (unless you negotiate some)
If you do not agree, you may be made redundant, and would be entitled to compensation for the redundancy. In determining your eligibility for redundancy, they would examine whether you had a reasonable offer of alternative employment. Cutting your income by 25% would not be considered as a reasonable alternative.
It is unlikely. but if your employer was prepared to accept a change to the monday-friday as a temporary measure (and put this in writing), then your current contract would be suspended, and could be reinstated at a later time for the purposes of redundancy or any other rights that you have under it. This is complex though and would need decent representation by a Union and/or legal advice.
Bottom line though is that you don't have to agree to anything you don't want to, but once you do agree, there is little that can be done.