There's a sort of reverse technicality here. The government's wish is that the law is 250 watts already, but it has not been altered pending the EU also finishing their revisions of all associated two wheeler laws. Therefore the only reason the technical offence exists is administrative convenience, saving the potential waste of time of altering everything twice.
So although administratively the law says 200 watts, the spirit of the law says 250 watts, and ultimately the spirit of the law is what the courts use as a basis for judgement. That is how the courts have power over the administration in our free society, and using that facility is how the DfT are allowing 250 watts until the law is rewritten.
