Interesting but why should the UK have wanted to invade in (19)20s and 30s ?
Why not? One scenario would be a commie takeover in the House of Commons and an alliance with the Russians of the time.... The Red Scare of the 20s would have been in full swing.
At any rate, it always makes good military sense (and it's good training) to make invasion plans for your neighbors, no matter how unlikely it'd be to have to exercise them.
We were their major Naval rival in that period.