TFC for longhaul is not simply adding the two one ways, it is priced as a return. I think you need to look at what the TFC is if you price up the return booking, so JER-LHR-PEK/KIX-LHR all as one booking.
If you do that for a revenue booking you get the following:
Passenger Service Charge - United Kingdom £48.21
International Tourist Tax - Japan £7.30
Passenger Service Facilities Charge (Int'L) - Japan £20.40
Passenger Securities Service Charge - Japan £2.30
Carrier imposed charge £381.50
Total charges £459.71
exJapan bookings do have very low carrier surcharge, but only for bookings originating there. If your booking is a LON-TYO-LON booking then you do not benefit. You can get around this by booking two one ways, and if you read the 241 guide thread and the wiki you will see there is a way to do this even with a 241.