Been looking at the 2 for 1 offers and now am thinking it may be the way to go for us.

Please correct me here if I an misunderstanding:
If we get the train line issued Travelcards for 7 days, that's all we need for any of the 2 for 1 offers during those 7 days. So, if we want to go to Dover castle 1 day and use a 2 for 1 there; it is ok to, for example, take the National Express Bus instead of the train, showing the proper still valid paper London Travelcard along with our printed 2 for 1 coupon is all we need.

so we'll pay for the LHR to London tube on our own Sat. night, then go over to Victoria or some place to get our 2 for 1 eligible 7-day Travelcard on Sun. morning.