GTW is Holesov Airport Czech Republic - LGW (Gatwick) is not served by BA from NCL so I think LHR is only option for Ash666 to use avios to fly from NCL and transit via another airport.
I thought codeshare flights could be booked eg Aer Lingus via Dublin, Vuelling via Barcelona etc.., but this doesn't seem possible through BA Exec. site, nor via Avios (Airmiles old site, where BE/BA codeshare amongst others used to be bookable)