Mark 1,
There are no restrictions on microlight (NPPL M) training, although there may be a "recommendation" that airfields "should" meet certain British Microlight Aircraft Association standards. (details of those I thought were on their web site but I can't find them, a phone call will reveal all)
With the NPPL mess/fudge, theoretically it is possible to do all your NPPL SEP training on a microlight at an unlicencesed airfield except for the final hour and your test which would have to be done on a light aircraft at a licensed airfield.
Of course you can alos add rating - such as aeros or floats or taildragger - to a licence at an unlicensed airfield.
Very best wishes
Colin