In reply to marktrik:
The governing body in the UK is the British Stand Up Paddleboarding Association, although the BCU also seem to cover them, so those are good places to start:
https://www.bsupa.org.uk/
https://www.britishcanoeing.org.uk/
I've had BCU membership for years for both the liability insurance and the included waterways licence - all for £45 a year. In fact, mine's just renewed and I probably need to upgrade it to include my wife, now we have 2 paddleboards.
There's also some useful stuff on this website: https://www.aquaplanetsports.com They've currently got an offer on an online intro course for £20. If you sign up for their newsletter, they have a lot of stuff in there, too - although, obviously, they're trying to get you to buy their stuff.
With all the sh!t currently pouring into our rivers and sea, you've not exactly picked the best time to get into it, though, unless you have a handy clean lake nearby!