I think other posters have more experience with this than I do, but I'll share what I've done in first/biz to LHR. AA in first is in a 1-2-1 config (or it was on the plane we flew). BA in J class on the big A380 was 2-4-2, so you could get 3 seats together in the middle, but know that the 2 in the middle 4 will face one way and the 2 on the aisle will face the other (it's really weird). We did have a Mom sitting in those middle seats with a baby and a small child and when lowered it made a pretty big "bed" for them to all sleep.
I'd check really carefully (you tube videos of plane configs are great for this) because many planes we don't make it easy to take care of a child. The seats on some (AF comes to mind) are just far apart and you wouldn't be anywhere near the baby, which is probably why we never see little kids on those flights. The only time DD was a "lap baby" was when we were on a first class flight that wouldn't work with her in her own seat since she was 12 mos old and still needed a lot of care and attention.
I hope this helps a bit.
Glad you made a decision and I'm looking forward to hearing more about it!