Hell Let Loose WW2 shooter. It has a great in-game community, and they just released the British faction too (which is a bit rough around the edges right now, admittedly).

@sideone, this is which I most play since several Years. 100% free, Windows, Linux, Mac, one of the best free games ever.

Official trailer
https://youtu.be/brJqHnXmpgE
https://www.thedarkmod.com/main/