We have round trip tickets to London. What will the supplement cost if we want to return from Rome?

Answer 1

I think you will need to ask your travel agent. It does depend on what kind of ticket you have.

If it does cost a lot, try to get a cheap ticket for Rome-London. But make sure that you do have enough time to transfer, as with the cheap airlines you need to collect your luggage before you can check in at the long leg of the flight.

Added:
If you do not have tickets yet, search for open jaw tickets, or a connection flight with the same airline.
Most likely cheaper than buying a single ticket even on a cheap airline.