This trick only works when your first flight is from outside the UK and will only work on the desktop version of easyJet.com and not the mobile website or app.
When booking with easyJet, if the first flight of the booking starts from the uk, any other flights in the booking that don't originate in the uk automatically get a 15-20% discount.
E.g. If I book London - Malaga return, the flight from Malaga to London automatically gets 15-20% off as the first flight in the booking starts from the UK. Whereas if I book Malaga to London return, there is no 15-20% discount. (This means suppose every London to Malaga flight cost £100 each way, the Lon-Malaga-Lon flight will cost £185 total whilst the Malaga-Lon-Malaga flight will cost £200 but the trick below will reduce it to £185)
Now for the trick. When booking Malaga to London return, first add the return leg to the booking separately i.e. London to Malaga, then click add more flights, then add the outward flight i.e. Malaga to London, it will automatically switch the flights round (as if you've booked Malaga to London return), but it will still apply the 15-20% discount.
If one is booking a single journey, let's say, Marrakech to London, the way to get the 15-20% off is as follows:
Add any flight starting from the UK to your booking let's say Edinburgh to Paris, then click add more flights and add Marrakech to London (the discount is then applied) then remove the flight starting from the uk by clicking the cross in the box of the flight starting from the UK and the discount will remain (the flight price will go up slightly due to the booking fee but the discount will stay).