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I cannot comment on military dogs (2 or 4 legged!), but I will offer some observations on the photos.

From my (limited) knowledge of dog sleds. That harness is a simple step-in sled dog harness. So, I'd suppose this is not a guard or search dog. What the heck it was doing in UK is entirely supposition.

Sled dogs are scrappy work dogs with short lifespans; bred for running and kept for as long as they perform. Retirement is a bullet. No room for sentimentality or feeding an unproductive mouth. The breeds have changed a lot over the years as competitive 'mushers' look for the best animals to pull. Some guys doing the Iditerod (Alaska) race have tried to go with greyhounds and poodles - because they may give a competitive advantage. True Arctic-bred dogs are furry, short snouted beasts with no manners. They can be quite dangerous unless the musher and the lead dog dominate them. That dog looks very much like a 1940's German Shepherd, never mind the colours. After that, I out of ideas.