I am in Dorset not going to China until 2013 but wanted to reassure you about Heathrow. I am in my 60's and have travelled quite a lot and find Heathrow one of the best airports to get around. Yes it is huge and busy but the signs are very clear and easy to find so you should have no problem. I also found most the the employees very helpful if you ask for help.
Let me know how you enjoyed the trip.