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You can join British Canoeing for £45/year and it gives a licence for the member (rather than for each craft if you go via each waterway) and applies on 4500km of waterways.
https://members.britishcanoeing.org.uk/on-the-water-membership/
Paddleboarders are included. Basically any "unpowered open boat", from dragon boat to light inflatable. Open still applies to craft with splashdecks (kayak).
Pretty good when registration for a single named craft (you are required to display a small licence plate) for the Thames alone is 90p more per year than BritishCanoeing membership!
Waterways licence, I think it's called. Estuaries and the sea + some rivers are exempt. Some reservoirs charge a premium (thanks Anglia Water...) or you have to join a registered club at the reservoir.
https://waterways.org.uk/waterways/using-the-waterways/canoeing-paddle-boarding
I only found out when I was warned by someone nearby that they have been asked for theirs in the last couple of weeks on the Great Ouse.