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.
HALO:MCC >>>>>>> Interested in Online Customs, Squadmates for Ranked, and Co-Op Campaign. I am on 6am-4pm GMT on my nights off (Midnight-10am US Central). If you add me do it on XB1 please; my 360 friends list is nearly full.