I guess I really don't understand the Subarbush thing, whether in the US or UK. However, no f1 breeding, will make a purebred anything. It does indeed, take tons of horses, generations of breeding and more time and money than most humans have, to make a breed which is breeding true. Along the way, hundreds will probably have to be culled.
You can find many Gypsy Cobs in the UK, which are of dilute colouring. Those from reliable breeders, will have a known background and will have been DNA'd. I know of a few. However, I don't see how breeding to one, will make a foundation or otherwise, Sugarbush breed. I guess in my mind, whether correctly or not, I had the idea that Sugarbush horses were Appy coloured Percherons. Come to think of it, there are some very good Appy coloured Gypsies in the UK.