Easy to fit (push-fit connections)................
http://www.screwfix.com/p/surestop-stop-cock-push-fit-15mm/63195
They operate by utilising the power of the water main itself. So, as Dave said, if there's a big blowout somewhere, it won't work. In normal circumstances, though, they're fine.
Your problem is that your valve ....... is in a silly place. To site this thing, that may also be somewhere inaccessible. If you can find the pipe run coming FROM your stopcock, perhaps there'll be somewhere much easier to get at.
In that case, I'd just go for one of these.........
http://www.screwfix.com/p/pegler-ball-valve-blue-15mm/11085
Quarter-turn... easy to operate in a panic.
Also............. you could have one of these................
http://www.screwfix.com/p/surestop-remote-stop-cock-push-fit-15mm/48604
Same as the regular one, but with a remote switch/valve that can be sited 2 metres away from said silly position. It's NOT an electrical switch. It's connected by plastic tubes.