You will be arriving at Terminal 3, and departing from Terminal 1 or 4. Although you can choose to clear immigration and customs, you're probably better off just following the signs for connecting passengers. If your BA flight leaves from Terminal 4, you'll have to take a bus; if it departs from Terminal 1, I believe you just walk. In either case, you'll be able to use the BA business class lounge in either Terminal 1 or Terminal 4, which will be large, probably crowded (it usually is in the morning, and I'm assuming that you're arriving on an overnight flight), with a decent (but not great) food selection and a number of computer terminals available, which offer kind of slow highspeed internet access.