I have never tried to clear immigration at T5 without a passport (its probably possible), but I take my passport with me in case I ever need to get out. Getting on the train to C gates and telling the guards you are no longer flying is all that's required.

Similarly, if you change your mind at C gates and decide you want to fly again, there is a service tunnel which you can use to walk back to the A / B gates. I think it is staff only but they will let you through if you explain you made a mistake.