They don't really have penalty out of cover, they just forget how to hold and aim rifles when there isn't at least a tiny bush or twig near them.
Okay to clear some stuff up here Tommies accuracy will be the same no matter if you are in or out of cover. Out of cover you have increased cooldown and reload and when in cover you have decreased cooldown and reload.
The Bren is actually (slightly, still doing all the DPS formulas)
better than the LMG42 due to having much less dramatic accuracy drop off, less time before reloading, and just straight up better damage. Basically in the things that count the most in the DPS formula the Bren has aces.
Basically your take away should be that you will be beating out Grens 1 on 1 with a large amount of reliability if you don't attempt to play hackysack with them at butt sniffing range 24/7. Stay out at range and in cover and you will do fine.
Of course this doesn't even get into Tommies having legit scary received accuracy for a 5 man squad once you upgrade....