Just a quick updates on Oyster cards.
Was in London recently for 5 days. Asked about the Oyster card. The man at the Tube desk said it was not worthwhile for me to get it for 5 days as a visitor. He recommended getting a 1 day Zone 1-6 TC for day 1 and then buying TC as I needed them. The reason was because of the £3 deposit. He did say the deposit is waived if a 7 day TC is placed on the Oyster card.

Turns out he was right. My hotel was very central so I walked everywhere quite easily. Basically only bought a TC on 2 days.

Also had to buy a Zone 1-6 TC to get back to LHR. The Piccadilly Tube line was undergoing repairs so passengers were being shunted to HEX. Was told that only the paper TC could be used since that is what the ticket collectors were checking.