Dr Who’s TARDIS is essentially based on the Police Box which you can see spotted around London and the UK. There aren’t many left and many are replicas. There are also many smaller ones designed by Gilbert MacKenzie Trench, and the idea was that they were all connected with the local police station and anyone could run up and call the police from the telephone inside.
The larger ones, like the Police Box TARDIS outside Earls Court Station could also be used as mini offices for the policeman to fill out forms or even keep a criminal locked up whilst help arrived.
If the light was flashing on top of the box the policeman nearby would know that a call had come in to the local police hub and he was required.
For a map of all the remaining police boxes you can visit http://www.policeboxes.co.uk/police-boxes-today.html and most are blue, although originally there was a red one in Glasgow.