From what I remember reading it was trialled over about the last ten years, five fractions and no HT. It is suitable for low risk patients. The beeb reports were that it is as good or better than 37 or 20 fractions.
In theory it should be rolled out across the whole of the NHS, the problem would be that not all hospitals have the equipment, or if they have the equipment they are not operating it in the required mode at the moment.
I was treated in 2018 at the Christie using an Elekta VersaHD Linac. I read the product information leaflet, and it said it could do SABR, but on me they were just doing the normal low dose reasonably wide field RT.
I would guess that the Christie would have been involved in the trial along with the Royal Marsden. I would guess that by now they would be trained up to operate the machines in SABR mode which I believe is what is required for the five fraction course. I would guess that any hospital with the right equipment would now be switching to the new method, for suitable patients. I would guess that it will be ten years before all NHS hospitals have suitable Linacs.
As you can see I am only guessing as to how widespread the treatment is, but we are seeing it discussed on this site fairly often so it is definitely on the cards, unlike a lot of promising treatments which seem to fall by the wayside (Proton beam, immunotherapy, Lutitium 177 all good but never fully adopted).
I am going to guess that in a SRT situation no one knows exactly where the cancer is so a more wide beam lower dose and plenty of fractions approach would be more likely to hit the cells wherever they are.
As I like to point out I don't know anything other than what I read on the internet, and what I think I can deduce with common sense. However cancer does not always follow common sense logic.