that is terrible. if you are a half decent php guy you could expect 25k. if you were really good 30k. if you could demonstrate something useful like hadoop or c++ more.
what else are they offering? if you are getting stock, cost for moving and regular training sessions on growing skills like scalable, Haskell or big data stuff, then you might just as well stomach it.
if you have a CS degree and have experience with version control, coding in a team, and delivering you are being undrbumped.
come to London get a work permit, leave and get a real job with a startup that value it's team.