Well if you insist on LHR and BA then you might just get away with a couple of £70 returns on two separate tickets. If you went British Midland then you might get the same sort of deal. You could try BAs lower cost options by starting at LGW for the same routes. The key is trying to buy both return tickets together so that the change in LON and no extra tax applies. This might give you extra mileage or status points, depending upon your programme rules for discounted economy tickets.

I would be tempted to buy the two coupon minimum if you insist on the EDI to CDG direct as this is the killer price sector. I would then look at www.flybmi.com for the single ticket at low price on the London sectors or try easyJet from all other London airports for the LON EDI sector.

If you were really trying to save money, you might consider taking the overnight train from London sleeper service and thus save a hotel bill. That would then give you the two coupon EDI-CDG-LHR option.

The final option, marry someone with a staff discount and go standby....