The MOT test does not check the legality of a vehicle, it only checks that it is in a roadworthy condition at the time of the test.
As an example it's illegal to use red diesel in a road car unless you have proof that the relevant duty has been paid, but does the MOT tester know about this or even check for this? Nobody can expect an MOT tester to know all the in's and outs of type approval and what is a legal or illegal modification to a car, they are just testing that it is in a roadworthy condition as described in the MOT manual.
The relevant wording in the case of stick on plates is:
vertically or, where that is not reasonably practicable, in a position as close to the vertical as is reasonably practicable, and
Obviously on our cars it is possible to mount vertically because Lexus make this possible from the factory. I'm sure we can all guess the reasons why plates need to be mounted vertically and what you could be accused of trying to evade if not mounted vertically.
Don't get me wrong I'm not preaching or jumping on the soapbox here, especially if you look at some of the mods on my other cars, it's just this subject comes up from time to time on car forums and actually the law is quite clear.