Bluestar, I had quite a lot of experience with my Mum, who was a wheelchair user. Mostly Aberdeen, LHR T5 transfer, Madrid and Malaga. Best overall: Aberdeen, with Malaga a close second. Madrid was good, but a bit complex and LHR T5 last.
My overriding comment: expect the unexpected!
Specifically about LHR T5: they do have wheelchair assistance, which 'should' come automatically based on the booking. I always asked for wheelchair return to aircraft steps* (technically the holding point on the airbridge, where hand to hold baggage is held when boarding) because it gives you more flexibility. (*On check in ask for a spare blank tag as outstations sometimes don't have them.) You will typically be asked to remain seated until other pax have disembarked. Almost always the assistance person will turn up with their wheelchair at the aircraft door as they dont know that you have a wheelchair with you. They can use this to get you to the holding point. Here things vary: sometimes the wheelchair turns up promptly and sometimes not; in many cases I had the pilot or co-pilot go to look for it. It can take a while. You need to look at flight connection timing in case you need to abandon your own chair to get to your next flight. (I always booked flights giving us 2-1/2 hours between flights just in case.) FYI the assistance person has a nifty app telling them your connection gate.
We had lounge access and again here things vary. The first couple of years about 50% of the time the assistance never turned up, so after that I always pushed the chair myself. I asked for assistance to meet us at the gate. Give yourself plenty of time if you're boarding at B or C gates. If it's a bus gate or arrival at a remote stand, they'll send an Highlift / Ambulift, but it does complicate things.
Once assistance didn't turn up at the gate and I said I'd push her down myself but was told that wasn't allowed - no idea why. Don't be shy in asking (politely!) to go to the head of any queues you find yourself in!
On the return assistance will help you through immigration (you won't really be able to use the automatic passport gates) and security. Until last year the assistance person could take you to the head of the security queue, but they stopped that and now you must queue with everyone else - "to make the process more efficient". I told them (strongly!) in person and by email what I thought of that, to which I got a big fat zero of a reply.
Cant say for the seats, as I am Gold and could book seats for free.
As I said, prepare for the unexpected and you should be fine. Good luck!