That won't buy you much, on the BoB menu or elsewhere. Not much of an incentive as a perk on a Flexi ticket, although the easyJet service concept from the beginning was based on it being paid for separately on board. I think they were the first to introduce that model in the UK and Europe. Personally I think it is fine for point to point flights, but I do not like it on network carriers feeding longhaul flights at their hubs. Who steps off a longhaul SWISS flight at Zurich and is happy to pay stupid prices for the catering on their connecting flight within Europe?