My understanding is you must have a means of knowing what the speed is in MPH. Most modern cars calibrated in KPH will have an inner scale in MPH and vice versa. That will satisfy the authorities. Classic cars may well have an exemption – best to check with your local garage that does MOT’s.
Failing that, you will have to find a sticker that you can stick on the speedo face to get it through the MOT.
Converting MPH to KPH is easy, just divide by 6 and add a zero. ie. 60 MPH = 100 KPH (roughly).
Converting KPH to MPH is even easier, just mutiply by 6 and knock a zero off ie. 60 KPH = 36MPH