I've done a little searching, and it seems that the 25kV system in the U.K. has always been 50Hz, fed from the National Grid.
What I must have had in the back of my mind is this old
6.6kV 25Hz system. From the same website:
The 15 kV a.c., 16 2/3 Hz, overhead system is used widely in Germany, Denmark and Austria.
In Germany the 16 2/3 Hz electricity supply comes from railway generating stations. Power is transmitted at 110 kV via overhead lines. The power is transformed from 110 kV to 15 kV at railway substations that feed the railway overhead system.
