Pre-JAR there were provisions for people that were monocular. I don't know whether that is still the case. Am sure that the DGAC would be helpful in finding out what the rules are.
I know for a fact that the FAA allows monocular people to fly and gain their licence. They have to have 20/20 vision and the refractive error may not be more than 3.5D. They have to do a flight test with an FAA examiner to demonstrate their ability to fly safely.
HTH.
FD