"In order to exercise the privileges of the ATPL(A) a valid Class 1 medical certificate shall be held."
Absolutely.
But if not excercising the privileges of the ATPL(A)...........?
What nick14 says about the entry on a Part-FCL licence is correct in that the lower licences are also explicitly listed.
And I reiterate my earlier question: what has JAR-FCL 1 got to do with the UK?