If you have: Radial tyres front and back AND The same size left and right at each endthen you're probably not breaching any regulations. See the MoT inspection guidance, but of course you can pass that and still be in breach of regulations. The effects on handling and braking, especially at the limit, are another matter. I think it's unwise to mix on the road unless you've explored them under all relevant conditions. Which is just about impossible. Jonathan