I'm found in all angels, and core of the Devil,
Angel is an anagram, while the "core" of Devil is V, which is simply an angle.
My forebears once dined 'cross the ol' English channel.
Angles, I assume
If fish is your wish, I can help fill your dish,
Anglers catch fish
I go well with pie and with freshly cut sandwich.
Cutting pies or sandwiches creates a wedge (angle) shape. Could also be a play on pi.
I wax and I wane with the tick of the hour,
The angles between hands on a clock
I only submit to my half-lunar ruler.
Maybe it's referring to a half-moon shaped protractor, not sure. (confirmed by OP in comments)
Don't argue with me, though the night may be long-
Some angles are obtuse, or "annoyingly insensitive or slow to understand".
Though sometimes I'm right, I will never be wrong.
There are right angles, but I've never heard of a wrong one.