CUK has a broader more inclusive vision of cycling than most member groups
I suspect this is part of the problem. It implies a lack of focus.
Compare and contrast with Audax UK, which is thriving (afaict) and one of the reasons for that is its very narrow focus - although there are some disagreements about the organisation's aims and objectives, it has a clearly defined reason for existing.
BC has the advantage of being the sport's official national governing body, which gives it a fairly clear purpose, even though it covers everything from track racing to MTB and BMX.
CUK, on the other hand, seems a bit vague. Many cyclists struggle to identify with it - which is partly a failure of marketing, since it does have something to offer most cyclists, even the MAMILs.
BC's GoRide activities are an example of the kind of thing CUK perhaps ought to be doing - they're not so tightly defined but they're clearly branded as a range of activities suitable for beginners, kids and less confident cyclists - and they're incredibly popular.