Tints wont make the car fail the MOT as its not on the test..
Ive had all my previous cars tinted, all have been darker than my current car and ive never had any problems, but then i live in the sticks and i dont see the police that often.
ANY tint on the front windows is now illegal, and if the police stop you for it you will need to remove them. However most tint places will now re-tint them for free if you want.
I think R20GRC only has a light tint on the 3 rear windows and nothing on the front (from the looks of it)