I have a return ticket booked for travel , during the September Tube strike , from Blackhorse Road to Three Bridges. The route is Victoria Line Tube to Finsbury Park and then Thameslink to Three Bridges. As I will be unable to use the Tube can I validly use the ticket on an alternative route ? I was considering taking the Overground from St James Street (Walthamstow) to Liverpool Street then walking to London Bridge to catch the Thameslink.