As mentioned above there is nothing specific for interline but there are loads of offers made by various train companies etc. it depends realy upon the timing of your visit. Ditto about the Oyster card for travel in London ... it also gives some 2 for 1 offers on eats and attractions. There is a thing called the London Pass but I'm not too sure if its good value for money. If you are thinking of travelling outside of London then think about hiring a car and seeing the countryside ... driving here is easy since we drive on the correct side of the road .