So people will point out, and rightly so, that you would be unlikely to be able to pass a NPPL GST in a Cessna 150 after just one hour in one, following 31 in a C42! You would have to allow yourself time to convert to a different aircraft.
And there are aspects to "group A" flying that are quite different to microlight flying - particularly the instrument flying and procedural aspects.
My experience is that around 10 hours is about right to turn a good microlight pilot into somebody who can pass an SSEA skill test.
Of course, no two pilots are the same - people have done it in 3 hours, whilst I've seen a couple who really struggled to make the extra step into group A flying.
G