How about a Chinese/Russian/Korean alliance? Bonded together by diplomatic Western aggression against their various states. China and North Korea are busy fighting the U.S in the Far East, heavily depleting Nato resources in Europe. Despite assurances of neutrality to the escalating crisis, Russia launches a blitzkrieg against Eastern Europe, and Scandinavia. 

By the time the RP takes place, Norway has just fallen, and Germany has been reduced to a smoke choked hell hole as Nato and Russia go toe-to-toe. With her army away on the European mainland, Britain then faces a surge attack by Russian forces using Norway as a leap pad. The Royal Navy scrambles to intervene, as does the TA and whatever remains of the army on Britain, but until they can congregate into a serious force, Scotland stands open to full blown invasion. 

Regional Scottish forces are obliterated in a joint airborne/naval operation from Russia, and all that stands between London and the Russian vanguard is the castle. 

The SAS, or whoever they are, have to hold off against the Russians until the British army mobilises its counter attack.

There's my two cents lol. Like I say though, go with whatever you want. Orcs? I can play orcs.