Tried to order off Backcountry.com but they inform me Montrail don't allow their stuff to be shipped outside the USA
Very telling that!
As I see it you have a couple of options
1) Email Montrail in the US and see what they have to say... they could be quite helpful!
2) Try and order from a smaller business i.e. a"mom & pop" type place as there less likey to care about the rules for the odd sale.
3) Google shows a couple of UK places offering the above item... unless its old info or there out of stock.
Forwarding companies have been mentioned but from what I've seen its only really economical to do with the more expensive items i.e embargo'd electronics.
Anyway good luck