However, you don't need to take the final segment back to LIS if you don’t want to. If you are on a J ticket (where stopovers are chargeable) and are worried about BA wanting to check your bags all the way through to LIS and you’re coming off a longhaul flight at LHR, then book your return segment back into OPO or FAO which go from LGW instead. In these cases, the LHR/LGW transit means they CAN'T through check your bags (the same principle works particularly well for AMS too, which is served from both airports). If you cannot book your last leg from the ‘other’ airport, you may want to book it for the next day (but less than 24 hours from arrival, so it is not treated as a stopover). This gives you far more leverage with BA if check-in at, say, BKK try to insist – incorrectly - that you must check your bag to your final destination.