Do all the above offer similar levels of insurance cover and assistance?
If that's all you're interested in there may be better offers outside of cycling organisations, you may already have adequate PL insurance, it's often bundled up with various other things such as household policies. That cycling organisations sell so many memberships on it's benefit is a bit of a wheeze, the cost is minimal as is the risk.
Another benefit of BC or CUK membership is the agreement with their tied solicitors not to charge the success fee to members who have successful personal injuries claims. Hopefully you'll never need it, if you do it could save to £1,000's. You might also find this elsewhere, I had it with a union membership, I don't think any other cycling organisations offer it (Clarion offer a discount from 25% to 15%, I don't think LCC have any such benefit)
I'm a member of Cycling UK, I get enough benefit to warrant the subs, but I support the aims enough that I'd probably be a member even if I didn't.