Can’t think of any good reason to counterfeit these whether now by the Chinese or back then by a contemporary Brit.
Wear, plus there is a plus or minus tolerance at all mints. I don't know what it is for British pennies of that date, but I would not be surprised if .4 gram were within tolerance.
I found cents that are supposed to weigh 3.11 G and they'll weigh 3.0 to 3.2 so I believe your coins with intolerance I don't know why there's would be any different than ours.IMO Dave