USAflyer, I do a similar transfer fairly regularly with my Mum, who is in a wheelchair. Our routing is Aberdeen to Madrid, so UK Domestic to European, and vice versa. We've had quite a few of the Madrid flights leaving from T5B and T5C, so a little bit the same to what you will do. As CWS says, 90 minutes should be adequate, as long as everything goes OK. If you were lucky enough to have a T5B to T5B connection (or T5C to T5C) that would help a lot. However I always book connections with around 2 hours, just to be on the safe side. The BA crew typically ask you to wait until all other pax have disembarked, so it does add some time. (It's amazing just how long it seems to take to deplane an A320 or A321!). Of course if you are time-pressed, they would likely be allowed off first.

One thing to slightly speed things up would be to NOT ask for your own wheelchair to be 'returned to aircraft' door - i.e. book it through to final destination. Returning the wheelchair to the aircraft door can take some time - and the assistance people always have their own wheelchair. One other tip - if Omniserve (the assistance provider) are busy, they may ask once you are in the terminal 'if you want to take it from here' - assuming I will push my Mum. I always decline, as they will help you get to the front of any queues.

We can use the BA lounges, but, for the departing flight based on some past, poor experiences, I always take my Mum to the gate myself, and arrange to have assistance meet us at the gate. BA will typically not allow me to push her all the way to the aircraft door.

Good luck!