Determine gate for departing Heathrow flight

I don't know about the other terminals, because I don't visit them often enough, but for Terminal 5 the gate is normally known to BA about 1.5 hours in advance of departure. I say normally, because if there's disruption (weather, inbound flight is late etc) things can change and it won't be confirmed. However, most cases (let's say at least 80%), BA will know at checkin time.

T5 is a little special, because it has 3 parts to it, T5A (directly attached landside), T5B and T5C (which has no lounges). At checkin, if you ask, they can almost always tell you which bit of T5 your flight will leave from, so you know which lounge to head for (assuming you have lounge access). They won't always tell you the exact gate then, however, if you ask when you check into the lounge they'll either tell you the gate, or tell you when they can tell you the gate. The lounge staff are normally able to tell you the gate number at least 30 minutes before it shows on the board. (They have to look it up in a different system though, it doesn't flash up when they scan your boarding card to let you in)

So, my advice is to ask about the gate at checkin, and see if they can look it up then. If not, clear security, then ask either the lounge staff (if you have access), or the airport information (if not). They ought to be able to look it up on the airport system and tell you.

For most parts of Heathrow, heading to the wrong bit of the terminal just means a 10-15 minute walk. For some bits, it can mean having to re-clear security and wait for a shuttle train, so it's worth double checking before you say head to T5C only to find your flight goes from T5A....