Then what did the EU directive change?
Metric was legal in the UK from 1896.
Most people and businesses continued to use imperial.
In 1995 metric was made compulsory for prepackaged goods.
In 2000 metric was made compulsory for loose good.
That is when the “switch” happened in practice. I remember it well.
There was going to be a brief period when imperial could be used as a “supplementary indicator” before being outlawed altogether in 2009.
Because it was going to be outlawed, most businesses switched to metric only.
In 2007 the EU said we could keep supplementary indicators.