Network rail have just advertised for apprentices, have a look on the website.
London Underground, are going through change, everything is moving back to London undergorund control and moving away from the likes of metronet.
You need a few certs before you can even go doen there,
entry to work permit is the main one, then after that depending on what your doing, your company will/should put you through the ones you need as there are hundreds of them, all depending on what you are doing!!!