Suggest a compromise of having Friday's off but only losing 15%.

take the 15% as the 4 day week equals 20% wage cut. you would gain an extra day off a week but lose an extra 5% pay cut too.
however another thought is if you have travelling costs and could probably claw back that other 5% and more over the year by working 1 less day??
hopefully you will still have a job when its all decided.
good luck.

If the difference between the 20% and 15% is not critical the day off may prove best IF you use it wisely and can manage to find another job in a more secure firm.

However, which do you think the employer would prefer you to take, if they even have a preference? Might be best to go with what they want as if they have to lay people off later they may keep those who have been more amenable.

Either way, a difficult position for you, your colleagues and employer. Good luck.

4 days id choose.

eg if you earn 400, on a 4 days would be 320, but 15% loss would be 340, would it be worth working that extra day for 20 (quarter of a days pay in reality for a full day), youd lose that in travel, lunch tax and NI id guess.

Also do Tax credits come into it ??

Also would save your employer a stack in electricity / power if thats a big money chewer for them ?