The pH makes no difference to corrosion - it will change with temperature, for a start (I can’t be bothered to figure out where it will go - sorry), but an acidic pH is not necessarily a good thing for brewing coffee (which is itself acidic).
As you point out, Lockhills is more likely to produce scale (as it has higher GH), which is not good. Silica scale comes in several varieties, but all require Mg or Ca to be present, as well as SiOx silicate ions (which I cannot find concentration for in either Lockhills or Volvic information labels - Volvic reports a 32 ppm for ‘silica’, but it’s not clear what that is).
Lockhills also has higher concentrations of chloride, sulphate and nitrate ions, all of which are potentially corrosive.
So it seems Lockhills ‘loses out’ for both corrosion and scale potential. Maybe it tastes better, but I would suggest that the main reason why it seems to be preferred by people in the UK is this: