Does anyone know were i can buy a humane dog trap, like the ones for rats ect?

I also live in the Uk but unfortunately I don't know of any 'trap sellers', maybe you could try tempting it in some sort of pet carrier with food or calling the RSPCA or SSPCA (depending on where you live), calling either would be useful as they have the equipment and resources to get the dog to safety and find a home, good luck in catching it!