There have been times when the Brits have been doing it pretty tough, and where stretching the life of tyres by regrooving them might have made a deal of sense. The Great Depression, followed by WWII, followed by a rather lengthy period following the war where was struggling with the aftermath of a protracted war, peacetime industry was struggling to get back on its feet and the country was having to pay enormous debts incurred during WWII (and substantial amounts still being paid off from WWI. There was rationing of various commodities until the 1950s, though I'm not sure when tyre rationing stopped.
Besides, as has been said, regrooving is not uncommon outside the UK. There was a time when racing "wets" were made by hand-cutting slicks, for example, and I've seen regrooved truck tyres too.
