If you are looking for the third party insurance and are a member of Audax UK, you can take Cycling UK affiliate membership which is cheaper than their normal rate - it includes the insurance but (I think, need to look it up, it's not super obvious on their website) excludes voting rights and the magazine. I think plenty of other organisations and clubs have the same deal. AUK members can contact me on membership@audax.uk and I can reveal the secret code!
Alternatively you could consider London Cycling Campaign (LCC), especially if you live anywhere near London. They are a bit more expensive than CUK but they genuinely do loads of campaigning work for cycling, especially the urban and utility variety.