Yes, the specified voltages are phase-to-phase. If you find a pair of binoculars and a pole in a suitably accessible location, that's the voltage you'll see listed on the rating plate of the transformers.
So far as I'm aware, the U.K. system either feeds into the HV lines with a wye secondary as you say, or there might be some delta connections with associated balancing windings to keep the system referenced and symmetrical to earth. Thus the phase-to-ground voltage will be at the usual Vph-ph./ SQRT(3).
At least that's how I understand the network to be arranged.
