the Air Navigation Order 2005 is the statutory instrument covering this matter:
(4) A person shall not be entitled to act as a member of the flight crew of an aircraft registered in the United Kingdom if he knows or suspects that his physical or mental condition renders him temporarily or permanently unfit to perform such functions or to act in such capacity.
The certificate (if issued) would therefore be effectively suspended. Procuring a certificate by non disclosure or concealment of a known condition would lay the offender open to possible criminal charges as well as the withdrawal of whatever benefit they had wrongly procured by virtue of that action. Therefore all a bit pointless.
I think most sensible people would agree that anyone considering this type of fraud would be eminently unsuitable for the position they were seeking.