My problem is that there is currently no way they can be ridden legally

Cars can do 100mph and people speed all the time and ignore traffic laws. Are you calling on a ban or speed limits for cars?

I've been learning to ride a bicycle for the my new work commute and have been brushing up on the laws and highway code. I was kind of shocked to learn riding on pavements is indeed illegal unless a sign specifies, since you see people do it all the time with no consequences.

I agree that E-assist bikes that don't exceed normal cycle speeds should likely be left alone (general education on cycling needs improvement but that's another matter) and the modified / faster ones absolutely ~~need more scrutiny and possibly a motorcycle license or equivilent.~~

EDIT: Classic didn't read the article moment:

Police say it is illegal to ride the bikes without a licence, insurance and vehicle tax and that they are classed as motorcycles.

Sergeant Gareth Davies said: "While it's not illegal to own an e-bike with an electrical assistance or power output exceeding 25 kph and 250W respectively, you can't ride it on the public highway as a regular bike without registering and insuring it like a moped.
"This includes both off-road and road rights, such as byways and bridleways. You can only ride unregistered and uninsured electric bikes on private land with the landowner’s permission.