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Surely it is possible to programme any location as a waypoint using either Lat/Long or OS grid refs? Or would that be too time-consuming?
Sure, but indeed time-consuming and error prone. Particularly since you'll want the pencil line on the map to correspond to the magenta line on the GPS, normally. Drawing lines from airfield to airfield is easy.
The Garmin 296 has the useful "Edit on Map" facility. Having programmed BKP to Top Farm press "Edit on Map" and move magenta line slightly east away from the Panshanger ATZ which also keeps you clear of the Luton and Stansted CTRs. Takes but seconds.