Keep in mind one important thing which can save you some money.
If LHR is your connecting point to go with your airline to your final destination you should not pay the departure taxes as long as you show your boarding pass to the gate agt.
Ex
CDG/LHR on BA
LHR/ORD on AA if you are a AA employee you will be considered as a through pax.
In order to be valid you must NOT make a stopover of more than 24h.
Same thing applies in NRT or any point where taxes are included in the ticket price.
I guess it should be the same for NW.