If your stepdad, an experienced farmer,thought the animals looked ok to slaughter, then I'm sure they were. As stated, the abbattoir will probably be less than impressed by a dexter because of the low volume of meat you end up with, as compared to a more commercial animal.
We had a young dexter bull slaughtered a few years ago. He was no where near fat, but turned nasty, so we ate him. And he was delicious.
There was not a lot of meat in comparison to one of our Herefords, but then you don't expect a lot from a small animal. The main problem because he was slaughtered early was the lack of fat covering, so the butcher couldn't hang him for long as the meat dried out. But it was very nice meat all the same.