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.
Landed on exactly 500,000/500,000gs with exactly 18500 achievements and exactly 500 games. All 100% including DLC. Was going hard the first half of January to reach that goal! Finished it on 1-19-2016. Final achievement: Completion Serenity in Nero.