Ok iv'e experimented pretty recently;
BP near me does not give you the cash price, however according to a rep. at BP HQ it depends on the the location bec. each one sets their own prices and policies.
However Shell near me does give the price for gift cards, and I was told by the station attendant (for whatever thats worth) that machines are set at HQ so it would seem that this might apply for other shell stations also.