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 for a search exist.
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" behavior that should, according to police codes of practice, make a search unnecessary.
It's not a good idea to discuss the case with the police until you have spoken privately with your lawyer, who you should ask to be present when you are being interviewed.
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