Haven’t checked the Greek docs, but to supply the UK reference:
CAP 493
S3 Ch1 12 (Visual Approach) .3: “Within controlled airspace,
standard separation shall be effected between such aircraft and other IFR and/or Special VFR aircraft.”
edit: you (theoretically at least) cannot force an IFR pilot to apply their own separation, but as long as they’re agreeable it is a perfectly valid procedure. In the US AIM it states the pilots responsibility is to inform ATC
promptly (and prompt isn’t defined anywhere) if they are unable for any reason to apply visual separation. I’d tend to argue that 14,000ft before you’ve even been asked to apply it is fairly prompt notice.