I'm just back from the UK and flew BA Premium Economy both ways from Canada.
You do have to pay extra to book the seat you want.
It was pretty good, on an Airbus 350.
It was 2-4-2 seating.
The seats do have a very weird legrest thing that only comes out to a 45 degree angle, so it's pretty much useless.
I found it just got in the way of my legs and bag underneath the seat in front of me and was more irritating than useful.
The seats themselves were spacious and comfy, with a thick quilted cotton blanket and pillow supplied.
The IFE was the best I've seen on a plane in years, with literally hundreds of choices of audio and visual entertainment.
They give you very tight-fitting headphones; not comfy, so bring your own.
The food at dinner was ok, not the best I've ever eaten, but palatable.
They did not have my first choice and I was in the second row of P.E.
The "wake-up" snack was disgusting on both flights...a sort of long dead red pepper in an over microwaved soggy pastry thing.
I would recommend bringing some snacks with you if food matters to you when you fly.
I had flown P.E. on Air Canada to Italy earlier in 2023, and it was far superior to BA's service.
If you haven't been to Heathrow in a while, it's bigger than ever and I walked 1.2 miles from landing to getting to the Underground Station to get into Central London.