*see UPDATE below* 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 £245 (€283). Very limited availability in last quarter of 2017 though.. This is including all charges and fees and checked luggage..
*UPDATE*.: Available once again now with travel dates over Christmas Holidays..
As mentioned very limited options are available between November and December 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
15. - 27.11.
22.11. - 11.12.
6. - 16.12.
13. - 23.12.
20. - 30.12.
Travel samples from Paris
18.11. - 9.12.
20.11. - 11.12.
2. - 16.12.
14. - 23.12.
21. - 30.12.
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 £245 (€283) here. In this case you would fly from Italy to Japan on 20th of December. Your return flight from Asia back to UK would be on 30th of December 2017.