Use the method posted by Xcess: Put a Russian on the map and group the Brits (or Yanks if you prefer) to that Russian as the group leader. But, set his probabilitiy of presence to zero (the slider all the way to the left). When the mission starts the Russian will not exist, but all the units grouped to him will be on east side (even though they are west units).
As for U.S. vs. UK in war......lets us first keep in mind that we have been very friendly to each other for about 190 years!!! ;D
If not for the help of France during the American Revolution, we would not have "kicked England's rears". As for the the war of 1812, most historians regard it as a stale mate. We were victorious in that we maintianed our sovereignty, but England was victorious in that they defended their Canadian colonies (otherwise Canada might have become part of the U.S.!!). The Treaty of Ghent was signed because both sides ultimately realized that the loss of life was not worth the potential territorial gains (as if loss of life is EVER worth territory!!!)
Here are a links to read more...
http://en.wikipedia.org/wiki/War_of_1812http://en.wikipedia.org/wiki/American_Revolution