I thought universal wage was supposed to work like universal basic basic income, the trials of which around the world have varied massively. There was a trial that seemed to involve also cutting the equivalent of PIP etc. (I'm trying to remember where it was - I think Ottawa, but I'd need to check). But it hasn't been done like that everywhere. Introduced well in some areas, it has shown promise.
In some ways, it can be seen as being like a tax allowance. After all, why should we have tax allowances? Why allow us to earn a certain amount or get a certain amount of capital gain without paying for it?
Or what about use of public services? There are lots of public services we get without having to pay for them, e.g. because of exemptions arising from age, disability, impoverishment, temporary residence etc. (Though I'm sure there are plenty of politicians and profiteers who'd like to change that.)
There are places in the world where they provide collectively, without payment by individual, what one could call a 'ration' that could be seen as an equivalent of a bit of UBI. Depending on where you are in the world, you might be able to benefit without payment unless you can afford to chip into collective taxation from 'free' water, wifi, education, healthcare, grazing land, foraged food, drainage, fire service, public transport, shelter, legal representation...
We have some of those, e.g. a lot of healthcare and emergency services, although successive governments have been privatising as many of those that they can.
However, in relation to what could be done in the way of UBI or universal wage or similar, power to pay extra benefits is devolved, but to my knowledge power to cut existing benefits isn't. So I don't see this Welsh proposal as being about cutting disability benefits.
That being said, if you asked me whether I thought Boris and his mob would introduce UBI if they thought it could be used to sneak in cuts to disability benefits/support, then I'd say yes, I think they'd do anything they could to pursue 'small state' and that I now, at the risk of being seen as melodramatic and paranoid, see them as eugenecist.
As for your scepticism about the £1.8 billion, I think you're mistaken. I think he will introduce it. Of course, he'll probably use it as an excuse to cut at least three times as much in other benefits...oh, I see what you mean...