There's lots of things that can cause uneven tyre wear,it also depends on whether its the inner or outer edge of the tyre.From leaking shocks,worn suspension bushes,tyre pressures,toe/camber settings,wrong size wheels.....etc
Also i seen a car go through the NCT two weeks ago and the tyre sidewalls were in a bad state but the owner put it through anyway and it passed.If you have good thread on 3/4 surface area of the tyre and depending on the wear it might pass.