Fans of an open jaw tickets have some good option to explore Japan and visit Rome, Paris or London on your way. Based on Russian flag carrier Aeroflot you can book open jaw flights on route Rome / Paris - Tokyo - London already for £280 (€312). Very limited availability in first quarter of 2017 though.. This is including all charges and fees and checked luggage..
As mentioned very limited options are available mainly in February and March 2017. Flights with Aeroflot have one short layover in Moscow. Below you can refer to booking as well as travel date samples.
Booking samples
Travel samples from Rome
22.2. - 4.3.
1.3. - 11.1.
8. - 18.3.
Travel samples from Paris
2. - 18.2.
20.2. - 4.3.
2. - 11.3.
9. - 18.3.
If you need an accommodation in Rome, Paris, London or Tokyo we recommend Booking.com or consider following promo codes that offer Hotels.com, Hotelopia or Venere.
Booking sample of cheap open jaw flights Rome / Paris - Tokyo - London from £280 (€312) here. In this case you would fly from Italy to Japan on 8th of March. Your return flight from Asia back to UK would be on 18th of March 2017.