And, to answer your question, no, agents don't expect you to be London-based, but they do expect you to be able to get to castings, some of which are at short notice and many of which are in London,
I think you said in an earlier post that you are in Rugby, so it's just 50 minutes on the West Coast Main Line to Euston. Virgin inter-city trains do it in 50 mins and even the London-Midland service has some trains that go straight through in about the same amount of time. We often pop down to London and back after school for castings or for work, it really is a doddle to get there.
If you are planning to do the journey frequently, it's worth exploring railcards. We travel on a friends and family railcard which makes the trip very affordable too.