Cost and time trade-offs
For value, the Piccadilly line wins outright. A single from central London to Terminal 5 is about £5.90 with an Oyster or contactless card, roughly a third of the Elizabeth line and a fraction of the Heathrow Express, and it runs direct with no change. The trade-off is speed: the Tube makes many stops and takes about 50 minutes, against roughly 35 minutes on the Elizabeth line and 21 minutes on the Heathrow Express from Paddington.