I just flew back to the US last month with our then 6 month old. It was WAY better than I expected and we're planning to go back for Christmas now. I brought 3 normal sized bottles (the 260ml ones) and had water in them for formula. I put more water in than we needed. At Heathrow, I was apologetically asked to take a small taste out of 2/3 of the bottles to prove it was water. We did and all was fine. We also brought a few unopened pouches of food (over 100ml each -- like 120 or so) and I put them in a clear ziplock bag and no attention was paid to them whatsoever.
On the way back from the US, I got a bit more scrutiny for those same 3 bottles and was initially told to dump them, and I pushed back and said that I knew I was allowed to bring water for formula and another TSA agent came and took over. Instead of asking me to drink from them, they whisked them away and put them into some sort of scanning device (without opening them) and handed them back, satisfied. I did keep my eyes on those bottles at all times, lest they open them. Again, same food situation and no attention was paid at all. I completely forgot and didn't declare the food on landing in the US.
We flew Virgin and got a sky cot and on the way to the US, it was barely used, but our little guy slept 50% of the way back on the red eye.
We brought some small toys and a book or two, but he was so excited to look around at all of the people, not much got touched.
Dress in or bring layers for your little person! On the way over, the flight was freezing and we ended up putting a hooded sweatshirt on top of pajamas as well as a blanket. It was so cold! On the way back, we were better prepared and put him in warmer PJs with a sweater and a hat.
Like I said, it was way better than we expected. I was dreading it and it wasn't exactly relaxing, but it was tolerable!