Once stopped, a police officer can ask to see your driving licence, insurance certificate or MOT certificate. If for any reason you don't have those documents with you you will have a week to take them to your local police station to prove you do own them.
If an officer believes that you are under the influence of alcohol or drugs, they can either ask you to take a drug test, breath test or do a physical test (a field impairment test where you are asked to walk in and straight line, turn around and walk back).
If you fail to take the breath test or supply a sample of breath without a reasonable excuse, including a genuine physical or mental condition, you can be arrested. If you are non white with medical condition you get beaten up until you have a seizure.