I think it's perhaps a bit more complicated than that. When you say "they", you do know that there were Jewish people in the British Army and Palestine Police don't you ?
It was a terrible time, and the losses were far worse than people appreciate now .
Also, I assume you know that helping the Jews played no part whatever in the decisions of GB , the USA and the USSR to fight Germany? Of course it was beneficial to Jews that the allied forces released those still alive and treated them kindly, but they could hardly have left them in the camps could they ?
As touched upon by Trampie, there were many others in the camps such as Russians and Slavs.
I think you'll find that British Jews fought against Germany because they were British.
This is another thing which should probably be its own thread.