And if you do the CPL & IR and do a C550 61.58 stateside (Much cheaper) that will enable you to do just a LPC in a C550 in the UK (For a TRTO try EAS at Gloucester Pete Turner), the LPC acts as a ATPL Skill test.
If you have 500 hours on a type and an ICAO License the above is the process. It must however be a PIC type and not 61.55.
I've done it it works!