Let's look at some of the vowels of the two languages:
The German vowels depicted here (in green) are:
/e:/ as in Beet,
/ɛ/ as in Bett,
/ɛ:/ as in Ähre,
/a/ as in Bann,
/a:/ as in Bahn.
In General British we have these vowels (in blue):
/e/ as in bet,
/æ/ as in bat,
/ɑ:/ as in bard.
1. English /æ/ does not exist in German; the nearest one is short or long /a:/ as in <Bann> and <Bahn>.
2. German /ε/ lies between English /e/ and /æ/; there's no short /e/ in German, only a long /e:/ as in <Beet> and a long /ɛ:/ as in <Ähre>.
3. German <Welle> and <wähle> are pronounced /ε/ and /ε:/ respectively, so there is no qualitative distinction.
When I try to talk my German students into using the /æ/ I sometimes can't avoid the impression that some of them have the feeling that they exaggerate the /æ/ - that some other person speaks through them - that I try to make a fool of them. Be it as it may - it's hard work to convince those doubting Thomases. And even when I finally manage to persuade (a few of) them it's still a long and winding and lonesome road till the /æ/ comes naturally out of their mouths.
The quality of these vowels of German and English is a tricky thing. More on it later.