I always assumed it's because tommies are tough mothafucka's that cost less per man than grenadiers do(while being quite a bit more survivable), have access to a cheap dual-purpose upgrade and are overall a good infantry unit.
grenadier is 60 mp per man
tommies is 51 mp per man.
The grenadier rifle is far superior to the lee enfield. The grendier is probably the best rifle in the game barring the g43(or sniper rifle) the PE can buy extra for. Lee enfield rifle is about the level of the volks kar.
The tommies have 65 hp with soldier armor. Best case scenario they're up against volks kar which push their effective hp to 100. less extreme is the gren kar which push the Tommy's effective hp to 86 hp. Worst case you're against flame thrower or stuka.
Tommies does have the advantage of a fifth member which pushes their survivability against sniper or the pziv's low aoe shell, although British faction as well get chew out by sniper. Against high AOE attack (like nebel, or mortar, or tiger shell), the extra man doesn't do much.
Even the basic LT alone grant a whopping 25% accuracy bonus to the tommies' weapon , with the captain providing 10% damage protection. Tommies is a different beast with the Officers around. And then there's the heroic charge.
The main thing about the bren tommies is that the bren is effective at long range, especially with the LT bonus. The squad can stand at distance where they take less damage but still deals alot of damage. the Flip side is the bren is pretty mediocre at short range.
The main problem is that tommy squad without all those buff are not worth the 255mp. They're not a throw away unit like the volks or rifles are. It's like stocking your entire armies with storm or grenadiers. Sure the stormies and grenadier are nice but you still need throw away units. People you can just throw their lives away.
