I don't know about mixing your own as I just feed bagged stuff for ease, though I must say I think I've read it wrong at £200 a half?! Sounds an awful lot!
We don't feed growers, all ours get normal Massey Breedsure which are sow nuts (I think, the other product is called turbo grow and we definitely don't want that!), no more than 1.5kg a day each topped up with pony carrots and whatever else they root up and what people bring in from their gardens and/or a share of the browse that's cut for the goats. The two we've got at the minute seem to enjoy hay whereas others have enjoyed a slice of straw each a couple of times a week. This is how we feed any rare breed pigs that we have, we have had pink commercial pigs once before which were fed growers pellets and got to 80kg by the age of 4 months which I found staggering!