If you can, I strongly suggest you switch your AA flight to BA! (I assume your tickets are flexible.) In-flight service will probably be better, but most importantly, if you arrive on AA and depart on BA, you'll have a terminal change from 3 to 4 which is quite a distance in the bus; and your luggage will also have to make the same change which will increase its chance of getting lost. If you fly with BA on both sectors you'll be arriving and departing from the same terminal.

BA has a 747 departure from ORD at 17:40 arriving into LHR at 07:20 (as well as another one an hour earlier). The return flight on a 777 leaves LHR 11:40 and arrives ORD 14:10.

IMHO F in a 747 is much nicer (smaller cabin in the "nose" section so more private, no through traffic, etc). Detailed reviews and trip reports elsewhere on this forum.

For your other questions:

1. Do you eat fish? In BA F there's a wide selection of meals anyway and you can eat what you want when you want. If you don't eat meat or fish, it would be worth letting them know in advance.

2. If you only have 3 hours the chances are the T4 spa will be fully booked for treatments although you'll get a shower. Plenty of lounges to relax in - you have the Concorde Room (for ticketed F pax only), the regular F lounge at gate 10 (avoid! crowded and gloomy), or the Terraces at gate 1a which are light and spacious. There's also a separate business centre with PCs, fax and printer if you need it.

3. don't know I'm afraid.