Sorry to be a little late on the game here, but I only just picked up details of this offer. Shell Drivers Club have circulated an email offering 20% discount on British Airways marketed and operated flights in economy class (World Traveller) and premium economy (World Traveller plus).
The destinations on offer are:
Rio de Janeiro
Dallas Fort Worth
To be eligible for the discount, your flight must depart from London Heathrow, you need to book your ticket by 24th March 2014, for flights with an outbound journey between 1st May 2014 to 30th June 2014.
The discount code you will need to apply is ShellMARZH and and there is a minimum spend of either £450 or £475 (the page linked above quotes both prices.
Be wary of this particular clause in the Terms and Conditions:
The Promotional Code entitles Eligible Participants to a 20% discount per person with a £475 min spend in the British Airways World Traveller cabin (economy) and World Traveller Plus cabin (premium economy) including taxes, fees and carrier charges on flights to the following airports listed using airport codes –NBO, HKG, ICN, MEX, GIG, SEA, YVR, DFW, SIN, CTU, PEK, DXB, AUH, MCT departing and returning from London Heathrow only. The following selling classes only are eligible for the discount O,Q,N,V,S,L,M,K,H,B,Y ( WT) and W,E,T,P (WTP) To identify the selling class of a flight, click on the flight number. BA will not be responsible for flights booked in the wrong class. The discount is applicable to bookings for children and infants, when contained on the same booking as an adult.
Which implies the staff discounted tickets (“Hotlines”) are not eligible as they book into selling class G.