Although it is thin, it will take up a whole overhead locker, using the space allocated to 5 other passengers. I would imagine that even if you checked in online and headed directly to the gate, the gate staff would take it off you. I would ensure that it is securely packed so as to survive being thrown around the baggage system, as the Gatwick ground staff are pretty hot on cabin baggage, and now we are into school holidays, flights will be packed to the rafters.