I think I mentioned above that failure to comply with the terms of a right of way is trespass, not a crime. The landowner can take civil action against a trespasser, but they won't get anywhere if they call the police and ask them to book you.
If the owners of towpaths want to make rules or ask people to get cards, then it is only a courtesy to comply if you want to use the facility. But on the other hand, I don't see how they could enforce it if you didn't want to comply. If you were to cause an accident though, you would probably have committed a crime. Cycling safely and considerately is the key.
A landowner cannot rescind a right of way that exists in the definitive map maintained by the local authority. They can ask the local authority to abolish the right of way, but that would generally be agreed only where there is reliable evidence that the right of way has fallen into disuse over a long period. It would be very rare for a Rights of Way Committee to agree to an abolition (or reduction) of a right of way because the landowner complained that it was being abused.
