I don't see Halo ever being a non exclusive title. It is Microsoft's Mario.
Re: the UK, unfortunately you're right in that they aren't available as yet for the UK, although I'm sure some will crop up on the 'grey market' for resale to Europe, albeit with a price gouge from the seller looking to turn a profit.
400,000gs reached 1-13-15. Used D4 to get it (get it, it has 4 in the name) due to my disgust with how Halo: MCC has been handled. Achievement used for 400k milestone: Business as Usual.