Highly recommend Chiltern Railways for passengers travelling from Birmingham and West Midlands to London, and also London Midland, can get some brilliant priced fares with both operators... Everyone always thinks of Virgin Trains and their fares are usually far higher, unless you get a bargain advance fare. Yes granted, train journey times are a little longer,but the savings on tickets can be worth it. Walk on fares are also substantially cheaper.
Yep - spot on: "... And once you’re there, you can make unlimited journeys on London’s buses and tubes too! ..."
The image above seems to say this offer is a family travelcard. So being a travelcard would include unlimited onward travel from Marylebone on all rail and bus services in zone 1-6. It would also get you a 1/3 discount on the river boat services. This makes the ticket much better value than any of the advance tickets mentioned above, and also the non restrictive nature of the return journey would be attractive. PS voted cold because of "be gentle- it's my first post"
As usual you can get cheaper advance single tickets for a specific train time. I was in London a few weeks ago and the two single journeys were about £15. Don't forget you also can have the london 2-4-1 days out if you pick up a leaflet at marylebone. You can also try a ticket split website as occasionally breaking up a trip to more single tickets and bring this down too.