At this time of coronavirus it's valid for a company to look at alternatives to keep afloat, reduced hours is one, redundancy is another, shutting the company completely is yet another.
It's important to keep in mind that we in the UK are going to go through a very hard time, obviously this is speculation, but given past events and what's currently happening with financial markets it's not hard to come to this type of conclusion. Between on-going Brexit issues and now coronavirus we're going to be hit hard. Some companies have already planned redundancy exercises based on "weathering the storm" of Brexit, now this will no doubt push others to have to do the same. Even after the virus is gone companies will still have to try and recover for the huge financial losses they'll no doubt take.
Remember though, the grass isn't always greener - and at a new employer your legal rights revert to practically nothing. You'll no doubt have a probation period, and even then it's 2 years continued service before you'd have redundancy rights against discrimination (as an example) should that happen between now and then, your sick leave would be back at the starting level (albeit with certain government provisions in place - for now). It's a gamble to leave, it's a gamble to stay.
Ultimately you have to weigh up the pros and cons that are personal to you.
On the point about working full time hours but half day wages - a company can request this during this situation, however it should be deemed as overtime which you would be entitled to be paid for (catch 22 there). The only way I'd be willing to do that is if I believed the company could come out of this intact and with written and signed guarantees that this money would be paid back, or holidays given of equal value, after it turns around - don't get this verbally, as you most likely wouldn't see it. Also don't forget that your holiday pay is based on hours worked, so working reduced hours will reduce your yearly holiday entitlement - and after an Employment Tribunal last year holiday pay is also to be given for overtime, which you would miss out on if you work hours for free.