As already said, by 44', I'm not sure that it was optional, and I think that most were "called up", pretty sure my grandfather was, and he was in Burma.
June the 6th was a pretty quiet day for most of our chaps, most of our guys stolled off the ships with little actual opposition on day 1, unlike the US, who called pasted on Omaha beach.
Got a lot more tricky after that with a lot of scrapping against the Germans until Patton broke through and the whole German front line collapsed.
As for really nasty times, here's a few:
- For us, being amongst the parachute regiments flown into Arnhem. Probably the biggest losses that the British took in Northern europe
- Being on the receiving end of the German response at the "Bulge", when the Germans attempted their last key counterattack of WW2 in the West
- Fighting almost anywhere on the Eastern front, on either side
- Being on the receiving end of the firebombing raids in Japan. Many people mention Dresden, and for good reason. Picture a similar bombing raid over Tokyo, where the houses are made of wood and not stone, and the fire defenses are completely inadequate. The result were more deaths in one night than from either of the atomic attacks. I know that the Japanese were pretty nasty to our POWs in the Far East, but at least they didn't go around systematically firebombing major cities.