Just renewed my insurance with Haggerty (cheapest for the cover needed), rang back and queried the legality of carrying a pillion without pillion insurance.
They confirmed that it is not illegal to carry a passenger without their cover, just that in an accident which was my fault, any harm to my passenger would not be covered. They didn't say it but it's true that the passenger could sue me however.
Then they said that there is no additional premium for carrying passengers (!) So I asked why they asked the question. I didn't fully understand the answer but did not want to string out the conversation.
You can take the odd passenger without passenger cover. News to me.