You only require to have a cpl to command a dual crew ops SPA aircraft in the UK.
The rules changed with JAR that to gain a ATPL you had to have passed a LPC in a multicrew aircraft after you had all the experience requirements. So it would be nigh on impossible to get people qualified to sit in the LHS if they started off in the right without sending them off for a useless type rating on something bigger.
The initial type rating the pilots train for is a single pilot rating on the aircraft. Shortly followed by a LPC in right hand seat if thats the seat they are going to, to give them a multicrew IR in that aircraft.
You can get some quite large aircraft operating under these rules the largest I can think of is the Beech 1900D