If your vehicle has a defect which makes it potentially unsafe, you may be in breach of the law, specifically the Road Vehicles (Construction & Use) Regulations 1986. These regulations cover deficiencies in tyres and brakes, and other mechanical problems with a vehicle.
Typical examples include driving with:
You will be in breach of the Regulations if your tyres and brakes are defective, or you overload your car, van or lorry. Driving whilst using a phone also falls under these regulations, because of the safety implications.
Breaching these Regulations is made a criminal offence under the Road Traffic Act 1988. For many of these offences, the police will offer you the chance to pay a fine of £100 and take 3 penalty points on your licence.