UK approval
here´s my question:
do I need a UK CAA approval to take an OPC from a JAR licenced pilot flying on an UK AOC?
Yes you do, especially in the UK!
It is NOT simply enough to be acceptable to the operator if it is a UK operator.
Typically you need to notify the CAA who is your sponsor for this - normally the Company.
Then, you need to do a CRMI course with an approved training facility (cost vary), then do a TRE standardization course e.g. at FSI in EGLF (about 3000GBP) and then you have to do a checkride with somebody, which is supervised by a inspector of CAA trainingstandards. (His cost about 1000GBP in addition to the plane or sim + to get him there....)
After all this, the CAA will issue you a certificate and the Company can list you in their OM-D as TRI/TRE to do OPC's
(for just a LPC you don't need to do the above)