Carphone warehouse has a good tariff, about £9 per month, virtually unlimited texts, fairly low data but shouldn't really need to hammer the internet on journey to/from school and could always use free wi-fi if need internet at other times.
The good thing is it is capped so wont get runaway bill, and if it all gets used up few days before end of month, might teach some planning and rationing which they will need later in life.
One other thing, for an 11 year old, dont get a fancy phone, if they get it out to take a call, or text someone in some areas (including some bus routes) it will be an invitation to muggers. Giving them something expensive and desirable is indirectly assisting your child to be a victim of crime.
If they want a smartphone something like a galaxy ace is good enough. Remember most muggings etc are done by opportunists, and they will go for easy target or rich pickings, doubt if you've ever heard of a battered basic phone being stolen.