London: use an oyster card and save money
The introduction of the Oystercard for the Underground in London is a lifesaver for the tourist stuck for change, and what’s more, it saves you a bomb on short trips around town. For example, GBP3.00 for a trip in Zone 1 (Central London) becomes GBP1.50.
Buy your Oystercard at the ticket office at the Heathrow Underground ticket office and you’ll save 50p on your ticket into town (GBP3.50). The Oystercard requires an initial deposit of GBP3.00 plus however much you want to put onto it. Then it’s simply a matter of passing it over the yellow disk at the entrance and exits to the stations; the relevant amount is automatically deducted.
They give you the chance to register your card, but it’s not a requirement. The advantage is that you can get a new one if you lose it or it’s stolen.