Most RECs standardised on 11kV and 33kV but we still operate at 3.3kV in some areas, and NEDL (North East) still have some network operating at 20kV, but I don't know if this includes any OH.
I'm not sure whereabouts, but according to the PoCo there is still a small amount of 6.6kV in my region (East Anglia).
