[lil_cain]

UK was legally obliged to by the EU. See the "metric martyrs" for how weirdly controversial this all was.

[onion2k]

Not quite. The EU directive said that governments should if they wanted pass a law to say metric units should be displayed. The UK government chose to ratify that law, but with the caveat that imperial units could be displayed as well if shops wanted to display them (and most did).

At no point was it ever illegal do display the old units. There were no martyrs; there were only idiots.