I plan to use Oyster instead of paper Travelcard when travelling around London because on arrival at Heathrow I can just top up my Oyster and go away to Central London by tube.
However, I still want to use 2for1 offer. I found that train ticket from West Hampsted Thameslink where I intend to stay to Kentish Town would cost just 1,70 GBP. Can I buy such a ticket and use it with my 2for1 voucher to get discount? Another question is the date. Can I buy the train ticket at the first day of my stay and go to the attraction few days later?