I have a 1 year old who has figured out how to get out of a child seat and does it frequently en route. When I say has figured it out, I mean she unclips the lock and struggles out of the seat.
I'm sure I saw something in the Sunday Times driving supplement about this a while ago. I think you can get something that fits over the lock and is properly childproof but can be opened by you quickly in case of an emergency. If you're near a halfords I'd checked there as they are pretty good for most car related products either that or check somewhere like ebay as nearly everything you could put in a car is on sale there and at the least it'd give you a few ideas.