When I saw that they were about to go outside at the end of the movie, I thought it might be better to wait until things are quieter. Even if the British soldiers see you surrendering and hold fire, there is always the possibility of a stray round. Losing the German jacket would have been a good idea as well.
I imagine they hadn't gotten much sleep the previous night though, and were a little too eager to believe they were then safe. However, I do believe Leech suggested waiting.
This has bothered me too. If it was me, and I chose not to listen to Leech, y not simply crawl on the roof, hide wait for a patrol to come by, then yell 'HEY GUYS! WE'RE 2 BRITS UP HERE, DON'T SHOOT!'? At least take off the very German jacket, even the pants, walk around in just a regular shirt & undies. Or, get a huge white flag, not the hankie they were using. I just don't think that 1 last scene was realistic.
I agree completely and was quite put out when I saw this the first time as a kid. Now, of course, I see it for the complete twist that a movie full of twists needed to end on.
**SPOILER** And given the shenanigans going on back at HQ that endangered the team in the first place, even had they gotten so far as to be picked up by a British patrol, perhaps steps would have been taken to eliminate the last two survivors to prevent embarrassing questions from being asked.