It is not correct to say that you can not log P2 time in all a/c types certificated for single pilot operation, but it will be for some. The S76, AS365 and B212 are examples of types that are normally operated two pilot by many operators and as such would qualify. I don't have a definative answer, but I would say that types such as the AS355 and A109 would not. In any case, I very much doubt that any operator would use two pilots on a/c that small as it would prove prohibitive in terms of lost payload and extra operating cost.
It's a strange situation, and in many ways is changing with the introduction of JARs. Another anomoly is that the CAA will allow S76 (two pilot)time towards MCC exemtion, but not time in a Bandeirante or Twin Otter!
A co-pilot acting as a member of the flight crew must be line checked and Operator Proficiency Checked (OPC has replaced the base check).
I assume you are a CPL(H) holder? A PPL holder can't fly public transport anyway. I am a little puzzled by the query; do you have an offer of P2 time if you pay for a rating? If not, as I said above, buying a 355 rating will be of no value in loking for a P2 position, and a 76 or 365 rating would be rather expensive to say the least.
You may well find that Scotia are looking for newly qualified CPL(H) holders to fill co-pilot positions. If so, that would be a far more likely scenario for building time (700 ish hours a year).
Good luck, in any case!
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Another day in paradise