I don't think chagga grows that far south. I'm no expert but I think it's likely to be something else. The only way to tell is to cut it and see if it's orange inside. It looks far more likely to be a growth of some sort.
If you have found chagga that far south you are very lucky!
Yes, traditionally only one growing on birch is used as chaga tends to collect substances only produced by birch. I have not seen any studies of growths on other trees, russians only use birch too. I have at least one growing on rowan on my land.
As said above, break a little off and see if it's rust coloured inside. From your pics, i have my doubts it's chaga, especially so far south. The furthest south i've found it (so far) is near Maclesfield.