There are 4 terminals at London’s Heathrow Airport widely dispersed all over the airport.
In the centre of Heathrow Airport are Terminals 1 and 3 situated between the two main runways. Terminals 4 and 5 are on the perimeter roads several miles away.
The main Bus Station both for local, long distance and inter-airport buses at Heathrow is between Terminals 1 and 3. The Heathrow Express runs a shuttle train from Terminals 4 & 5 to Heathrow Central Rail Station beneath the central bus station. Some National Express bus services do stop at Terminal 4 or 5 after leaving the Central Bus Station, on their way to Gatwick Airport or places on the south coast like Southampton, but most others do not.
Terminals 1 and 3 also share an Underground Station which is beneath the bus station and the Heathrow Connect and Heathrow Express trains to London also leave from Heathrow Central, a station shared between Terminals 1 and 3.
At Terminals 1 and 3 only the taxi rank, hotel shuttle buses and long term car parking shuttles visit the terminals themselves.
From Terminals 1 and 3 there is a network of underground walkways that connect them with the bus and train stations.
Reckon on walking 10 minutes with luggage from either terminal to buying a ticket at one of the stations.
The routes are all luggage trolley friendly and you never venture outside in the open air.
The video clip below chronicles step by step your journey from both terminals to the stations.