Originally Posted by
andywine
Correct - my apologies. It’s 2 tickets and we must claim and recheck our bags.
So let's count back. Your bags must be checked in with BA at T5 at T-45 (you also must be through security by T-35). So 1hr 35 minutes left. Make sure to add in time for the transit from T3 to T5 which will depend on whether you use Heathrow Express (every 15 minutes) or the Tube (free but requires an Oyster card). I'm pessimistic but would leave 30 minutes for the terminal change. So down to one hour and 5 minutes left. Assuming you are able to use the e-gates (UK, US, EU passports) immigration should be fairly quick but you've also got to account for baggage claim time and the invariably long walk from the gate to immigration.
For my risk averse self it would be too short on separate tickets.