We have some of the best troops in the world, but there aren't many of them, could we win a war against a large country, like China, the USA or Russia without help?
On one hand, we won the Falkland war with ease, but that was mainly a Naval war, that ended after Argentina left the islands; we never had to invade the Large landmass. however, even with this lack of troops, we can hold our own back in britain pretty well, so it's entirely possible that we could win a war by fighting defensively untill the enemy has exhausted it's resources.