Cabbage Question

By selfsown, do you mean that the cabbage plants flowered and then produced seeds that scattered on the ground and germinated? If so, there is a possibility that they cross-pollinated with other plants in the same species that were flowering at the same time on other allotments.

The cabbage species is very variable and includes cauliflower, brussels sprouts, broccoli (heading and sprouting), some kinds of kale, kohl rabi, and probably a few other kinds I'm forgetting. So your plants will be edible, but maybe they won't form heads of cabbage like the parent plants. You might have some very interesting new types of brassicas.

I don't know. people don't usually move them when they are that big, but you could try moving a few of the smaller ones and see how they do. Cabbages like very firm soil, so tread all around them after moving them.

Yes. You can give them more space by eating some of the plants.

It depends how big the variety grows. From 1 to 2 feet is generally recommended, or 30-60 cm if you prefer metric.

I don't think that would be a problem if you give enough spacing between the flower and cabbage plants. And if they don't shade each other too much.

They are probably spring cabbage. I usually eat half as greens and allow the rest to grow on and heart up. The only way you can transplant them when they are bigger is to take a large rootball with them and give plenty of water.

Do you know what the parents were? Mind you, as said above, brassicas aren't fussy with their partners