Need a valid EASA for what?
The question was not in relation to EASA but in relation to the GCAA license..
You are not required to have an EASA license to fly a Single engine c172 in Europe, any ICAO license is valid for up to 28 days without a validation for VFR day Only.
The question I had was related to the GCAA ATPL M/E if this takes presidence over GCAA PPL single engine, and allows you to exercise the privilege of a single engine or if the GCAA needs to state S/E