During the day (say 10am to 3pm) it should be easily OK. I have done it in under 45 minutes to Terminal 4, and that involves a 10 minute bus ride whilst T3 is a lot shorter.

If you on BA in business or first class then you can use the FastTrack line in Flight Connections which should shave off 5-10 minutes. If the line is ludicrously long and you are not FastTrack, go to immigration, go landside and walk to T3 which should take 10 minutes at most.

The bigger issue is whether your hold baggage makes it or not but, again, realistically 90 minutes should be fine.