Capt. Vilo
You say you have passed the written exams of the ATPL, but that is not sufficient to be exempt from the Met & Nav unless you actually hold a UK Flight Crew Licence (PPL, CPL, ATPL). Even if you have a letter from FCL proving that you have passed, not good enough, mad I know, but thats the ruling from the CAA.
If you don't hold the licence, you will need to sit both a Law and Procedures exam as well as the Met & Nav paper.
Good luck in the exams