It is illegal to have a tread depth of less than 1.6mm around the entire circumference of the tyre for trailers and vehicles under 3,500kg gross weight. Vehicles over 3,500kg gross weight must have 1mm of tread depth over the same area. You can measure the depth with an inexpensive tyre depth gauge obtainable from most tyre and accessory outlets.
Ideally tyres should be changed prior to the legal limit as wet, corner and braking performance diminishes considerably once the tread depth falls below three millimetres.
In the UK the Road Vehicles (Construction and Use) Regulations specify the conditions a tyre must satisfy in order that it can be legally used on the road – this means the load index, speed rating and tyre type need to be suitable for the vehicle or trailer they are fitted to.