Anything which is defined as a single pilot aircraft.
Although I couldn't see a Cessna 150 however you write the SOP's getting by the flight ops inspector as a multicrew operation.
The only difference between the privileges of a CPL holder and an ATPL holder is that an ATPL holder is allowed to PIC multicrew certified aircraft.
To be honest getting the upgrade to ATPL in the UK is a function of totalling up the times in your log book, carrying out your normal currency LPC/OPC check and sending money to the CAA. Other countries operating under JAR it can be a completely different kettle of fish with external examiners required, oral examinations and sitting a IR test in the LHS with various command exercises thrown at you.