Originally Posted by
KARFA
To be honest the best source may be to have a look at wikipedia under the destinations tab for LHR and LGW, any BA destination from either BA hub is a BA mainline destination.
The only other addition to that would be BA's LCY-JFK service as this is also BA mainline. The rest of the flights from BA are operated by BA CityFlyer.
Another possible resource is
https://en.wikipedia.org/wiki/List_o...s_destinations - the lists on Wikipedia's airline and airport pages are usually kept pretty up-to-date.
For using a BA companion voucher, I think that most if not all LCY flights ought to be OK. I don't think that it necessarily has to be that strictly "mainline". Some of the vouchers exclude franchisees, but CityFlyer is not that. But it would help to know which voucher is involved.