Push To Talk, PTT, PoC in the UK

I've had a few phones now with Push to Talk, PTT or PoC options on them, but after a fairly intense search around i couldnt find much on this technology especially in the UK.

So has it died a death or been convieniently forgotten by UK network providers?

i'd would like to know more about the system and how it can be used (if at all) in the UK.

thanks