Why is the assumption that the issue is with MG UK rather than the dealers? Did MG promise the choice of spec and colour as part of the initial allocation of cars to your dealer from the first 1,000 being delivered?
From everything that I've seen MG made no promises other than to deliver a number of cars based on the dealers previous sales to each dealership. Perhaps there could/should have been a central ordering system to book spec and colour within that allocation, but I've heard no mention of that existing.
So whilst I understand your disappointment I'd suggest that the issue is with your dealer raising expectations unreasonably by allowing you to believe that they could command what spec and colours they would receive, and hence could pass on to you.