We are talking UK here - I assume
I think the situation with line checks is pretty clear. Line checks are not training or revalidation of a rating or licence. Line checks are checks on the line. the person doing the check need not be an instructor or TRE. Since lots of helicopter line flying involves flights to and from non-licensed sites, there is no problem with line checks.
Prior to JAR FCL, it was clear that any flight for a licence or rating flight test needed to be done at a licensed site (or government aerodrome) - it said so quite clearly in the requirements.
Since JAR FCL , nothing is clear. I am not quite current in training matters any more. It would be worth wading through LASORS. My recollection was that the CAA were trying to impose effectively the same requirement as pre-JAR FCL. Someone current in training may be able to help.