Lasors put it quite clearly on the first page dealing with ATPL criteria...
Copilot time on JAR/FAR23 aircraft operated by two crew as per the company AOC, counts towards the 500 Hrs multi-crew requirement for the issue an ATPL (for sure if issued by the UK CAA!!).
Nevertheless, you'll need a JAR25 TR on top of these 500 Hrs multi-crew on JAR23 to unfreeze your ATPL.
refer to LASORS, it's crystal clear!