It's pretty well agreed that the trial in Finland worked:
https://www.newscientist.com/article/2242937-universal-basic...
Id welcome a trial that included paying for it. That said, I don't think it would make a big difference. The UK (I'm a brit) spends enough on welfare to just give everyone £5000 pa (including kids) at the same price. We could just start there and make zero changes to tax policy...
If the average is £5K and many are receiving £0, it must be the case that some are receiving much more than £5K in benefits. If you normalize everyone at £5K, what happens to those who were previously receiving some multiple of that? (I’m presuming that £5K is an insufficient amount to meet what society has previously considered their need for support was.)
In essence, you’d be proposing a UBI for ordinary UK citizens to be funded by those who are currently receiving welfare [over £5K]. That would be unpopular with proponents and opponents of UBI I think.
(If half or more of that £5K is in staff and overhead and you were willing to dismantle those organizations as soon as it was clear UBI “would work”, this might be workable. I don’t know how much is direct benefits vs overheads [including eligibility and fraud checks, which would be dramatically reduced under UBI].)
The Finnish trial "worked" in the trivial sense that unemployed people given an extra £2k a year on top of any other income they might earn were marginally better off. It didn't work in the sense that it was considered worth raising taxes to pay for it to be extended
Abstract of the final Report of the Finnish experiment, mostly in finnish though. https://julkaisut.valtioneuvosto.fi/bitstream/handle/10024/1...