The Law in the UK
Defected or badly worn tyres can shorten your life if you don't care about your life care about others.
The legal minimum tread depth for car and similar tyres in the United Kingdom and the European Community is 1.6mm throughout a continuous band comprising the central three quarters of the breadth of tread and round the entire other circumference of the tyre.
Most car type tyres have tread wear indicators, usually at least six small ribs across the bottom of the main tread grooves, and when the tread surface becomes level with these ribs the tyre is at the legal limit and must be replaced.
Simple and cheap gauges are widely available to give an acceptable guide to tread depth. One of the features of tyre tread pattern design is to give good grip in wet conditions. Generally speaking available grip reduces as the tyre wears or as the road surface water depth increases and you should reduce speed in such conditions.
The Penalties for illegal tyres is very severe i.e. tyre tread depth below 1.6mm, mixture of cross ply or radial, different tyre sizes on the same axel, defected tyres, bulges, cord showing, rips etc, and also tyre pressures these factors can all result in 3 penalty points and a maximum £2500 per offence.
So four defected tyres could equate to loss of licence, loss of earnings, loss of life.