@Ruth890 I'm not fully up to speed with the devolved laws but if meeting in houses is illegal then it would surely be illegal to Airbnb a room in your house. It would be different if it were a long time lodger as they would be part of your household.
As to who would be breaking the law I am not sure if it is the guest, the host or both!
Anyway the contract between you and the guest has been 'frustrated' by the new law so a full refund is due to the guest and a penalty free cancellation should be allowed by Airbnb for the host.
Search frustrated contracts if you need more ammunition but I hope Airbnb will have got the hang of the law by now.