The jetstream 31/32 is a multicrew type aircraft. Under civilian rules it has never been a single pilot aircraft. So the LPC is valid for ATPL application. The french did try and convert it to single crew but it never came to pass.
You could fly it single crew but it wouldn't be very pleasant, I think the RAF and Navy did on their's but not sure if the cockpit setup was different.
So yes with jetstreams it is CPL or ATPL as co-pilot and ONLY an ATPL holder as PIC.