Hi Bob,
You'll be counting down the hours soon!
Right then in my USA experiences I've checked in and taken the same route as if I were travelling anywhere else in the world but I wanted to double check with our aviation team and this is the response....
Check-in in Belfast as you would normally do, as the ESTA is tied to your passport, if there are any problems they should be flagged at this point by the BA representative. If everything is in order (so your ESTA is still valid/you've not changed your passport) you will be given two boarding passes - one for the Belfast-Heathrow leg and one for your Heathrow-Denver journey. Check that you are given both of these!! You will arrive at Heathrow T1 and will need to make you way over to T5. Once there simply make your way to the gate.
I hope you have a wonderful time and look forward to your posts when you return.
Kindest regards,
Julia