Sounds unlikely from a logistics perspective. Checking in at YYZ you will be issued through boarding passes,including from LGW to FCO, beacuse that's what your reservation will say. If you choose to check thru only to LHR and then ask about FCO flights there, how will you make your LGW flight if you are unsuccesful?

Sounds like more stress than just hopping on the Speedlink to LGW.