There, rather unhelpfully!, seem to be two R value scales.
The Americans give R values in h·ft2·°F/Btu, and most other people use SI units (K·m2/W). The conversion factor is 5.68.
I think that the US numbers are usually used partly because bigger numbers mean better sales, and partly because it was Thermarest (Americans) who first started giving R values, and competitors like people to be able to compare.
I wouldn't like to rely on existing hiking boots, mostly because they would probably be sized too snugly for the cold. Adding extra socks will just make matters worse, with the circulation to the foot being restricted.
What you want is a relatively spacious fit, an insulated sole and a modest amount of leg/foot insulation. I used to use Canadian army mulkluks, which are basically a felt bootie, a rubber/canvas shell, and the insole, and which were good for inactive use down below -40 (aurora photies).
The trouble with just buying "snow boots" in the UK is that very many of them are fashion items with no expectation of use outside of normal UK winter temperatures, so there's a lot of chaff to filter out of any search. I'd go for local purchase when I get there, or a recognised North American or German/central European brand such as Sorel, TNF, Meindl etc. Of course, something cheaper and visually suitable may be OK, but you can't see the insole, so it would be a bit of a lottery.