under PACE the following has to happen:
when approached by a PC he/she must: -
state thier name
state the name of any colleuges.
show I.D
state the Police station inwhich they work at.
Give you the reason why the approached you.
You have NO OBLIGATION to give your name, age, address of any information UNLESS the PC arrests you. Then you are obliged.
If in the event of your arrest - the PC must tell you why your being arrested and also state your rights such as right to silence, legal aid blah blah blah.
This applies to 99% of circumstances, others circumstances, Im not sure about.