Looking at the code, it appears that it wasn't originally an easyJet spec model. (easyJet's Boeing customer code is -3V, G-OSLH was a -6Q). It was probably destined to go somewhere else and already registered or soon-to-be-registered, and then was taken by easyJet at the last minute. It only stayed in the fleet for two years, much shorter than the other now-departed -700s.
(Edited to add... 6Q is Bouillion Aircraft Leasing. Maybe a lease to cover for slower-than-expected delivery in the rest of the fleet or something of that nature? I don't know, just a guess.)