I think they've probably all been replaced by now, TBH, although others may know differently. They were used extensively by the Southern Railway from the late 1920s onwards and also by the LNER for the Stratford - Shenfield and York area resignalling schemes, which were completed by BR in 1949 and 1951 respectively. There may be odd ones still in use as distant signals in otherwise mechanically-signalled areas such as Bognor, Littlehampton or Eastbourne - Hastings.
More modern signals use bulbs such as the SL35 with separate circuits for the main and auxiliary, that raise an alarm when the main blows. However, colour light signals of that vintage used bulbs such as the SL17. With such signals, both main and auxiliary filaments were both lit together. The auxiliary filament was lower wattage, so in theory should last longer, and wasn't quite in the optical focus, so appeared dimmer. Drivers were supposed to report a signal that appeared dimmer than usual, so that the bulb could be replaced before its auxillary filament blew and the lamp went out. I seem to recall that there was a collision caused by a SPAD somewhere south of Croydon due to a lamp failure of such a signal, that was a big driver for the Three Bridges resignalling to replace them.
Some of the heritage railways may have some survivors. The GCR (both N and S) seemed to have some heritage looking colour lights on my recent visit.
BR standardised on rectangular back boards for its signals. LT standardised on a more rounded lozenge shape. So whereas a signal with such a rounded lozenge backboard on BR would likely be quite old, I am not sure the same would necessarily apply on LT.