Cornish Cabbage Plants

Cornish Cabbage Plants is a family business that grows plants from seed for sale as young plants. They are different to most because the plants are field grown and sold at a much larger size than the commercial module grown ones. https://www.cornishcabbageplants.com/
You may remember that I visited a local organic veg grower earlier this year and I've just seen a video of him planting out a delivery of Curly kale plants from this company. The plants were much larger than anything I would replant. Apparently they have been pulled from the ground, not dug up, and are replanted by being dropped into dibbered holes in the ground, as one would do with leeks. 
Not sure if this link to Pip's video will work https://www.facebook.com/watch/garthview/
Having seen the size of these kale plants I would have no qualms about moving brassicas around the garden or of growing them to a decent size in a seed bed before moving them to their final positions. No more  puny little seedlings for me!

I have never grown brassica plants from other people since introducing club root to a plot by planting cabbages I was gifted!  All my brassicas are grown from seed or cuttings.

Interestingly, the Cabbage people say that:-

"Raising pH levels above 6.5 is essential. Placing sea sand or volcanic rock dust in the bottom of the hole when planting will lessen the likelihood of club root disease. "
https://www.cornishcabbageplants.com/plant-care

Interestingly, the Cabbage people say that:-

"Raising pH levels above 6.5 is essential. Placing sea sand or volcanic rock dust in the bottom of the hole when planting will lessen the likelihood of club root disease. "
https://www.cornishcabbageplants.com/plant-care

Tried 'Rockdust' once, can't say I was impressed and it was very expensive. I would have thought sea sand would be salt laden? (unless there is such a thing as washed sea sand?) , salt, as we know is toxic to plants?
It is well known that a mildly alkaline soil is beneficial to brassica growth.

Having said all this, if they have been going for some time without negative results, good luck to them!

I don't think either sand or rock dust will have much effect on the pH of soil. I've used rock dust in the past but don't see the point in it anymore, it can take decades for the minerals to be made plant available. If you haven't done a soil test you've no idea what your soil could be deficient in. Even if there is a deficiency there are much better ways of correcting it than rock dust.

You can definitely move brassicas but I think the stress of disturbing their root system like that would make them more likely to go to seed. When I've done it I try to keep as much of the root system intact as possible.