Just to clarify, I'm talking about an Apple Premium Reseller - but yeah totally agree with you. Extremely frustrating. To be fair I've ordered things from this reseller in the past as soon as they've been released and have typically received stuff only a few days to a week later than the mainland UK, but this sounds like it's going to be way off.
With regards to distributors, my understanding is that all Apple Resellers (including APR / Apple Premium Resellers) buy from one of a few distributors in the UK / Europe (this is what my reseller told me over the phone anyway), so it seems there is some sort of bottleneck, I assume Apple prioritising delivery of custom machines to their direct customers (which I would be if they sold to Jersey residents!).
Frustratingly, because my machine is a customer order I'm not able to get a refund / my deposit back so can't even cancel my order and buy elsewhere ;(