The cheapest way to the airport. The Heathrow Express is 35 minutes quicker for £20.10 more, which is not worth it at this party size.
Piccadilly line
Piccadilly · Transport for LondonPickup, hours & luggage
Boards at. Any Piccadilly line station in Zone 1, including Piccadilly Circus and Green Park
The same £5.90 on Oyster or a contactless card, from any Heathrow terminal, to any Zone 1 station, at any hour — TfL offers no off-peak fare on this journey at all. A paper single is £7.00. It counts towards the daily cap, which the Express does not.
Luggage. No stated limit, but it is a tube train and the Piccadilly line has no racks.
Hours. Timed from Green Park, for Terminals 2 & 3 and Terminal 5 (23:40 last on Sundays). Terminal 4 starts later, at 05:54 (07:00 Sundays), and stops earlier, at 23:18 (22:42 Sundays) — and has no Friday or Saturday Night Tube at all.