You'd have to ask the originator of the expression for their intended meaning. To me, it sounds like a general purpose form of abuse.
However, if we were going to be deconstructive about it one could start by asking what we know about wombles. What are they known for?
Basically, they wander unseen across Wimbledon common collecting rubbish (the everyday things that people leave behind.... or something like that). So generally, some kind of secretive public area activity.
Then consider our notions of 'cock'. Clearly it could relate to fowl or poultry, but equally to the male sexual organ (also used for urinating). Add those senses to our knowledge of wombles.... and the idea that is most alive to us, the sense we most clearly have is that of something akin to a.... chicken herder. QED.
A cockwomble is a chicken herder.
Whilst I like the deconstruction, I don't think the logic follows.
I completely agree with 'what we know about wombles'. However, I don't think the prefix 'cock' can logically be said to refer to what they are gathering/herding. If it was Redwomble we wouldn't assume they were gathering red items. If it were fatwomble we wouldn't assume they were gathering up bacon rind or the contents of the dripping tray from the grill. We'd assume we were talking about 'a womble that was red' and a 'womble that was fat' respectively.
Therefore, I think following deconstruction 'cockwomble' can only mean 'a womble that is a cock'.
Now interestingly this could follow to the other suggestion above about it being a metaphor for a 'boss womble' - after all the cockerel is, I believe, normally the alpha chicken in the hen house (I stand ready to be corrected if I speak rubbish here). However, I suspect that the vernacular meaning of 'cock' is more suited - I.e. 'You are a cockwomble' = 'You sir, are not just a womble, but a cock too'.
Now we just have to ask whether it really is offensive to simply be called a womble, or whether there's an element of racism (species-ism?) there...