It's not so much BOGOF at London Eye as it's mainly tourists coming from outside the country, so they're unlikely to be buying a box of Kellog's or bottle of Carex (and they only cover SEALIFE and the parks generally anywa).
It's the multi attraction tickets they want to flog. You can add another attraction for a tenner more, two more for £20 or £90 total for all attractions with the hop on/off bus thrown in too. Suddenly you've got people locked into Merlin attractions (or a bus company they have a stake in) for the duration of the day. Lot's of opportunity for additional sales of merch/food and drink!