Like I said, all he needs is Brad Pitt or Matt Damon. Wouldn't hurt if he is also Tarantino.
Although I accept that historical accuracy is the first casualty of just about any period film, there are a few basic tenets you stick with. The SS were the most virulent hate filled racist members of the german military, if you want a film that offends everybody, then make the SS into hero's, otherwise just stick with the regular German army, historically the Brandenburger regiment was the closest thing the Germans had outside the SS to a special forces unit. For the british you're probably going to want the parachute regiment who did in fact perform special ops behind enemy lines during the war, they even had a bad ass nickname given them by the germans a la Inglorious Baserts, the germans called them the red devils.
Anyways, as for the overall scenario, it is pretty ASB, but then again so is Inglorious Basterds, didn't stop it from being made or being successful. Although let me make a suggestion, you could give the SS a plan to start wreaking havok in berlin in an attempt to get the russians and americans to go to war with each other, and then get the british and german commando's join forces to stop the plan before "it's too late." That's the closest I could come to a "plausible" scenario and stick somewhat to your original premise.