Alcohol 25% by volume translates into 50 proof in Canada and U. S.
The better qualities of scotch, bourbon, "Canadian," etc. are 90-100 proof, or about double.
So the proper alcoholic must drink twice as much Pimms as the strong stuff to keep sozzled.

Thanks, Gnomad, for the explanation. The Pimms as sold in the bottle reminds me a bit about the much stronger "Southern Comfort" of North America, which also has spices and traces of liqueurs. Horrible stuff, but thar be them as drinks it.