GAME do not have the largest distribution in the UK, they just have the most stores, which they made certain of when they bought Gamestation. This may give them the largest physical distribution but that's it. However, given that a large part of game distribution is via Pre-order and Internet Sales, GAME are starting to struggle because they are losing their regular clientel. Companies like play, Amazon and Gamestop are heavily eating their chunk of the online sales market and GAME cannot compete due to their physical store overheads.
As such, GAME are spending a lot of money with companies to make sure they get either the only, or the best version of Collectors Editions, such as N7, or the Epic Edition of Gears3. The problem is, game bought pitiful numbers of the Collectors Edition, and not many of those were made available for the Online Sales. Store Ordered Preorders take priority when it comes to allocation and they ran out of copies a long time ago.
As for EA providing the exclusivity; apart from GAME most likely outbidding, GAME has physical stores and that does count for a lot when companies look at potential retailers, though this is becoming less and less the case. GAME has either misjudged the numbers that would purchase the N7 edition, or just didn't care as long as they were the only ones selling it. Either way, there's nothing to be done about it, especially this close to the release date, apart from complain.
I try not to use game any more due to appalling customer treatment and service, as demonstrated in this latest fiasco, and also they ruined Gamestation. However, when they have the monopoly on Collectors Editions, I have to resort to using them, sadly.
I didn't read every post, but if as it was preordered online, the OP will definitely get their money back as there was no delivery of goods and to not refund the entire cost would be against the Trading Standards Laws in the UK, especially the Sale of Goods Act.
I hear that Gamestop in Ireland still have some copies left....