Hi, Welcome to the forum, Ben.
As far as space goes, I don't think you can really give them too much (within reason). For example, My wife and I have a pair in a 300 gallon stock tank, which is about 5 feet wide and 6 feet long. This is a very spacious cage for the pair, but not what I would consider excessive. The smallest cage we will keep a pair of adults in is about 4 feet long, 18 inches wide, and about 20 inches high.
For breeding, you don't need a group, they will breed just fine in pairs. You can maintain pairs together all year, unless they start fighting, or one declines in health. I would not recomend caging the babies with the adults. I have never attemped it, and I have heard of this ending badly for other people.
Not sure where you may find an adult male chuck in the U.K.
Hopefully someone has one for you.

Hope this helps.
Take Care.
Nick (& Austin)

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