There are a lot of separate issues at play here, so we will try to deal with them in turn.

Your front and rear tyres differ in aspect ratio, which is not desirable but neither is it dangerous. It is more important that your tyres are identical across an axle, and in terms of size (unless the manufacturer specification dictates otherwise) rather than the same make, although of course if you have uni-directional tyres then all four should be the same in this respect.

The speed rating of a tyre not only reflects the maximum speed the tyre is safely capable of, but also its capability in respect of the vehicle performance in terms of cornering grip and traction, which is why high performance vehicles frequently have tyres rated higher than their maximum speed. It is not illegal to fit a tyre with a lower speed rating than recommended, but in the event of an accident your insurance company may check the tyre rating and subsequently not pay out on your claim. Ultimately it is your decision if you choose to fit a tyre with a speed rating lower than Y.

As far as the MoT is concerned, testers are required to check that the speed rating is in accordance with the minimum requirements, but this is for the class of vehicle rather than the specific model. However, that does not mean an MoT tester will not fail a car if it has tyres which are substantially below the required speed rating.