The principle of English (and Welsh) law that a person may not be tried for an offence for which they have already been tried in another jurisdiction (Which for these purposes would include Scotland). I cannot comment on the Scottish position.

This may be entered as a pleading by the defence, as in the case of Howard Marks (I forget the year, although for such an offence he would have been tried in the High Court), where accused of smuggling drugs into the UK (He had), he had been convicted by a Dutch court on a minor charge relating to the same offence.

Compare double jeopardy.