Definitely try and book window seats if that’s your thing. Especially on the 747. Book seats in front of, or behind the wing. Once I booked a seat on the wing of a KLM 772 and well, the view is a big ol wing lol.
The British 747 is legendary, but high hopes you are on a retrofitted one. 747s are starting to get long in the tooth so you may see some wear & tear but no big deal.
American 737 is a mixed bag. From what I’ve heard the new tighter cabin may have only been installed in their new Max aircraft but some have said its on their -800s as well. Nothing too special, don’t expect food on that flight since it’s a domestic one.
The 777 is a great aircraft all around. Nothing to really comment there. Maybe a little older but I’m sure a retrofit has already been done to those as well.
Biggest tip is be on top of the time changes. Tune your body to London time as soon as you get aboard.