All tyres must have at least 1.6mm tread, the legal UK minimum but you should change tyres well before they become this worn. Use a tread depth gauge to check the depth of the tread. It's a good idea to check the general condition of tyres. After jacking up each wheel in turn, look for any damage, bulges and anything that's got stuck in treads or sidewalls. If you find anything, call a tyre specialist for more advice.