I didn't realize these were from Britain. They have entirely different standards there, so you can pretty much disregard what I wrote about OEGBs as far as color and leg color go. I still think the coloring is mixed for the OEGBs, though, but would have to see the British poultry standard to be sure. I know that slate colored legs are acceptable in Australia and probably Britain, too.Thanks @JedJackson that's interesting.
Why do you think the breeds are mixed?
I found the ebay listings I bought the eggs from, which show some possible parents, at least they are some of the breeders birds.
https://www.ebay.co.uk/itm/6-Old-En...-/334030289963?_trksid=p2349624.m46890.l49286
and
https://www.ebay.co.uk/itm/6-Serama...-/334030289961?_trksid=p2349624.m46890.l49286
I can see some wheaten and BBR.
I wish I could find a good wildtype chicken colour chart to compare with.