It would change the handeling of the car, and this could lead to safty issues, but i know a lot of high end sports cars have smaller wheels on the front and different profiles. Front to back. This improvess the handeling. But they have been designed that way. It is not illigal or is it an mot failure, so feel free to do it, but be carfull driving until you get use to any change in the handeling.
The most important thing here is, as said safety if the car was ment to be like that it would have come out of the Factory with different tyre sizes, 2nd important Issue your Insurance if they have cause to find your car with incorrect tyre sizes that may have contrubuted to an accident BANG goes your cover also you could get sued by the other party.
as long as you dont swap the wheels from corner to corner.ie n/s/f to o/s/r so the size across the axel doesnt match should be ok if they arnt the same across the axel it will fail an mot..abs will be ok