I had a look at the feed mixing regs and I have a feeling it does not include people who buy bags of feed and mix them in a bucket just before giving it to the animal....eg a scoop of pony nuts and a scoop of oats to a pony.
I think it applies more to those who mix lorry loads in hoppers on farm....so consumers and the fsa know what was fed in the mix and in what quantity.
Now as for feeding goats a home mix it really depens if for meat or milk and remembering high yeilders need high protein.
I used to have goats producing over 1500kgs milk per lactation and some over 2000kgs and were winning at shows and milking competitions so had to feed accordingly. Others I raised for meat.
A good mix is alfalfa nuts, sugar beet nuts, flaked maize, flaked peas, rolled oats, rolled barley, linseed cake and a good goat mineral
Then I used to do a pick a mix application...more alfalfa and peas for the high yeilders, more maize and barley for the thin less for the fat, no beet for males as causes urinary tract stones, and linseed for all!
Good hay to complement, pref Alfalfa hay (Lucerne) fed ad lib.
I always aimed for a 20% protein mix for milkers and a 16% for the rest....hope that helps!