To be honest, river paddling in the UK is a SERIOUS pain.
Most of the waterways dont have access rights, and those that do vary depending on the fishing season.
Check out:
http://www.bcu.org.uk/ and see if you have a local club near you, they're normally by far the best for advice.
There's no legal requirement to wear any personal safety equipment when paddling, but you'd be a grade A idiot to paddle without a buoyancy aid even if you're in an open boat on flat water. Get one of these sharpish!
Also, fill the boats full of water and check for any leaks. Are they fibreglass or plastic?
Turn them over and hit them a few times with your fists. Hard. If they break, throw them away. Bear in mind on a river a boat may hit a rock or bankside, with your ~70+Kg in, plus the momentum. You dont want to end up in the middle of nowhere with a broken boat!
Oh, and dont try to do this on day 1

(yes, its me in there somewhere, river Dee @ Llangollen)