"but you did have to stay with it until that episode to get the joke. "
Not really - Schulz speaks English with his light German accent with the two British agents they kidnap in no-man's-land on the Dutch border in the very first episode.
The problem with his accent is that although very light, it is there, and he would have been 'spotted' upon his first words on his mission in England.
What puzzles me is that Schultz speaks without accent - and therefore in German - with the French Countess, although Schulz speaks fluent French.
If this were a serious drama production, I should prefer all characters to speak the correct language for the scene (using native speaking actors), without daft accents, and with switchable subtitles for monoglots. Schulz would speak accent-free English, too, if he had any hope of not being caught in England. But Private Schulz is a light comedy, so the solution chosen is quite acceptable.
Any comparison with Allo, Allo is unfair - that is high-camp hard-comedy, and the accents there are central to the show.
I assumed that his French accent was meant to be so good that he didn't have an accent (if that makes sense - he didn't sound German when he spoke French) whereas his English accent wasn't as good, and so he sounded German.
Sorry, I know you posted a while ago, but I've just finished watching the series - and thoroughly enjoyed it! - and I noticed your post.