I got tugged in central London the other day. Good cop & bad cop. Bad cop was all miffy about the small 9&1.5 plate. Good cop shooed him away, no fines being dispensed, and gave me the following kindly advice:

"say you've just been riding off-road and haven't put the street plates back on yet - you'll get a more sympathetic hearing"

for what it's worth, I give you that gem.