We only switched to selling by the litre in the early 90s (presumably for the sake of EU alignment), it was sold in Gallons until then. Expressing efficiency in MPG is just something that had "stuck" by then.
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.