you have the right to be "treated fairly and with respect" by the police

You can be stopped and searched if the police have a reasonable suspicion that you are in possession of controlled drugs.

Police can stop and question people whenever they wish. They are supposed to ask questions in order to decide whether or not grounds exist for a search.

If you are stopped, ask why and, at the end, ask for a record of the search.

If you give a satisfactory explanation for "suspicious" behaviour that should, according to police codes of practice, make a search unneccessary.

your rights on arrest
you have the right:

to know why you have been arrested
to be treated fairly and with respect by the police
to see the written codes governing your rights and how you are treated
to speak to the custody officer
to have someone notified of your arrest (but not to make the phone call yourself)
to consult with a solicitor privately

right to remain silence
You do not have to say anything to the police, and you should not be intimidated into answering questions. You should request a solicitor to visit you in the police station as soon as possible. You might have to wait, but it's always free.

It's not a good idea to discuss the case with the police until you have spoken privately with the solicitor, and you should ask the solicitor to be present when you are interviewed.

lock up
You cannot be locked up indefinitely. The police can only keep you for a certain period of time - normally a maximum of 24 hours (36 hours for a serious arrestable offence).

The police can sometimes keep you isolated and waiting in the cell to 'soften you up'. Above all else, keep calm. Make sure you know why you've been arrested, and that the correct time for your arrest is on the custody record.

source: Release: www.release.org.uk