The Oyster Card will work now on the Heathrow Express so you can use it there, just notice that will be costlier than taking the Underground . If you want to take the Heathrow Express, it may make sense to purchase less expensive advance tickets if you might get them to save money, but otherwise you can use your Oyster Card to pay on arrival. It may also be a pleasant time to walk through a few of the city’s fairly gardens or parks, or get uncrowded views of places from the surface like Buckingham Palace and the Houses of Parliament. I’d simply remember to check the forecast and have no matter you need for the day (e.g., rainjacket or umbrella) with you so you can benefit from the day. If you do get a travelcard, as you are primarily travelling in Zones 1 and a couple of, you’d only desire a travel card that covers those two zones. Then, whenever you travel outdoors of your travelcard zone, or when the travelcard is expired, it’ll just use the PAYG credit in your Oyster card.