Actually, the explanation is startlingly simple.

A horse-drawn Pickfords van was picking up a house lot in Bremen one fine morning a couple of centuries ago, when the German vowels climbed on board while the packers' backs were turned. Since this was before the days of Afghan refugees, there was no checking at the Channel ports, and the vowels duly found themselves in England, rather smelling of horse dung, a few days later.

Being good, upright, pragmatic German vowels, they checked out the local situation and changed their ways so that no one would notice that they had snuck in without formal entry papers. They had no intention of applying for political asylum since, in their view, they had just escaped from one (Germany was a rather fraught set of separate states in those days).

They decided their best course was to make themselves sound like the locals, so they did.

And that's all there was too it. Doesn't EVERYONE know about this?