The 555 only operates every 30 minutes, and it takes it nearly 20 minutes to get between terminals.

http://www.londonbusroutes.net/times/555_557.htm

There is no bus stop at T1. The buses operate from the Central Bus Station, which is in the middle between T1, T2 (as was), and T3. You use the passages which lead to the Underground to get to it, it is physically on top of the Underground station ticket office, there are stairs leading up.

Since security paranoia set in and various airport roads were closed to public traffic, the buses have to take a very convoluted route to get around the airport. The road mileage is probably 5 or more times what the distance is by train under the airport.