I can't speak for Brett, but I suspect that the same units go to his event because they're units that he knows and trusts. More power to him, I say. I fall in with IR226, even though they're not my home unit. They're a great bunch of guys, and have always been very welcoming.
That having been said, there are actually a fair number of guys I know who do 1940 BEF. They're just very loosely organized. I'm with a group of about 6 guys total who do this impression (we won best modern era impression at MTA 2009), and I know of about 3-4 others on the east coast who can also do it, and probably another half dozen or so more guys in the Midwest who might be willing to travel if asked. None of us have websites or any other way of telling the world we exist other than through word of mouth. The trick is getting all of these guys together in the same place at the same time. Same goes for the French.
Another trick is that some guys who do 1940 do multiple 1940 impressions, so they may be double counted in any census of guys who have 1940 impressions. A friend of mine and I both do 1940 British and German. I know another guy who does 1940 British and French. Another does 1940 Belgian and British. Etc. So, on paper, it looks like there are pretty good numbers for 1940, but maybe they're not as good as they look, yet. . .
I do like Rob's idea of posing this as a GWA-like time period. There are hundreds of guys who put together extensive WW1 impressions despite there only being two events per year. More than that, people are willing to travel from around the country, and sometimes the world, to participate. I rarely hear guys complaining that putting together a WW1 impression isn't worth doing because there are only a couple of events per year. People being unwilling to put together an impression because they can only use it once or twice a year just indicates to me that they don't care about the impression in the first place.
Finally, as if this note isn't long or random enough already, we have to be careful in asserting that anybody who can do late war German or British can do early war German or British. Having done early and late war for both, if you want to do them right, early war is an almost completely different impression. All too many people think that doing 1940 German just means wearing an M36 bluse. Similarly, too many people think that doing 1940 British means wearing P37 battledress and a respirator bag. While that's at least a step in the right direction, almost everything worn by soldiers on both sides changed between 1940 and 1944. I could write a dissertation on the differences in impressions for both countries. Then, of course, you have the French and Belgian impressions that all but disappeared after June 1940. These are all wonderfully enriching and enjoyable impressions to put together. Unfortunately, most people don't know what they're getting into when they try to put them together - assuming they want to do it correctly.
