Only accepting its own currency in settlement of tax liabilities is what provides a country’s currency with its value,
Therefore the UK cannot have MMT, can it?
Because HMRC will accept euros in settlement of a tax debt. In fact, it’ll accept artworks, collections of historical papers, all sorts of stuff.
We can go further again, there are many things which do have value which you cannot pay your taxes with. Bitcoin, the services of a lady of negotiable virtue and so on.
Why, it might even be true that the assertion itself isn’t true, eh?
The critique of a critique of MMT doesn’t get any better after that.