I'm not sure you are right there Malcolm. 1/6th gill (or 1/24 pint) seemed to be the standard "single" measure in England and Wales but I think that this is one area that did become metric.

Wherever I go, spirit is measured as 25ml (or double/ 50ml) which I think is larger than the old 1/6 measure. The only exception I can think of is a pub that has, within the last month, upped the measures to 35ml.