For off-roading enthusiasts, mud tyres are a popular choice. There is no legislation which state mud tyres are illegal, nor any that stops their usage on roads.
However, the legal requirements for tyres in the UK are set by the Department for Transport, and the regulations state that tyres must be of an appropriate size, load rating and speed rating for the vehicle they're fitted to.
Mud tyres may meet these criteria, but it's important to check that they are labelled with the correct marking (approved by an E11 symbol and no NHS (non- highway service) lettering) and that they are in good condition.
Some mud tyres, those with aggressive tread patterns, may not be suitable for use on the road as they can cause excessive noise, vibration and wear.