2+4 GWR HST's currently being converted for service between Penzance - Bristol/Cardiff which will be in service later in the year.
Wrong type of Castles I'm afraid. The proper Mk3 'Castle Class' were the NI Railways 450 Class Thumpers. They were built using a Mk3 Suburban unit body shell atop a Mk1 Chassis recycled from the former 70 Class Thumpers.
https://en.wikipedia.org/wiki/NIR_Class_450