If you are flying with British Airways to London Heathrow, you will arrive at Terminal 5. If you have a connection to make, I would allow at least 45 minutes from the moment you land to the time you can reasonably expect to be at the departure gate for your onward journey.
If you are flying from North America, you will probably land at the 'B' Gates. You will then catch the shuttle train to the main terminal building where you will have to go through 'Flight Connections', clearing customs and passport control. Expect to have your picture taken (no fingerprints) and to have to take off your shoes and put them through the X-Ray machine. You will then be in the main terminal area where the 'A' gates are.