My 2 or 3 traveling companions need to purchase 7-day tube/bus passes for our trip in September. Right now we don't know if there will be three of us traveling or four (will find out in the next few days).

I've got an Oyster card already with some amount remaining from my last trip, but would be willing to buy a travelcard at a train station upon arrival if there are four of us traveling, in order to take advantage of the 2-For-1 offers.

However - if there are only three of us traveling and the 2-for-1 is no longer an issue, should my friends buy 7-day travelcards for zones 1-2 loaded on an Oyster card? I'd like them to be able to take the tube from LHR into central London without having to buy another individual trip ticket. We'll probably take a car on the return trip. We will not be traveling outside of zone 2 except for the trip from LHR.

I know two of us could still do the 2F1 deal, and split the benefits three ways if we want to....