I use a combination of PAYG and 7 day travelcard on Oyster.
If I know I will be travelling 5 days or more at peak times it makes sense to get the 7 day travelcard, but if not I just use Oyster PAYG. If you still have a PAYG balance it remains on your card even if you buy a 7 day travelcard on the same Oyster card.
Best of all you can buy your 7 day travelcard online and activate it when you go through the barriers at FH.
Yes, I'm still relatively new to London and the flexibility of the public transport system is magnificent compared to the North East!