As far I know it’s not the band that matters but that the distance between your two open jaw airports is less than the distance between your outbound journeys origin and destination.
eg in your case DEL-BKK needs to be less than the distance between LHR-DEL
Also this type of booking can only be done via phone.
Last edited by TGLoyalty; Jan 8, 2020 at 2:08 pm