Why do you need a BCU membership?
Well, you've got an answer already. But I would want to be a member anyway, because they're the BCU and they do a lot of good work for the sport.
You're actually not covered for all rivers in England and Wales. Some of them are private, and access is up to whoever owns the right of navigation. Something the BCU are working on is improving access for paddlers.
But honestly, I've paddled all over the country, including places I shouldn't; no-one's ever asked me for my license. There's been such a cull of staff to monitor boat licensing, I could easily get away with not licensing my narrowboat if I were that way inclined. And the license for that is more than £700!