Do I need a passport to travel from Scotland to London? Is there any border patrol or immigration office that requires documents to re-enter the UK by train?
You do not need a passport, or indeed any form of ID, to travel between Scotland and England. Both countries are part of the UK, and as such there are no border controls between them. All you need is a ticket* for the train!
*Some tickets, like Interrail or possibly Britrail tourist passes, will require a passport to prove that you're not a British citizen, and thus eligible for the cheaper tourist passes.