The Senior Railcard offers reduced-rate travel for over 60s on most Standard and First Class rail fares, and also Advance fares as long as you book before you travel. It costs £30 but all fares are then discounted by one third.
To apply for a Senior Railcard go to any railway station with a valid UK driving licence, passport or birth certificate to prove that you're eligible. To buy online, you'll need a valid UK driving licence or passport.
Travel in London itself is free courtesy of your Freedom Pass