The planes they use from Gatwick are a bit tired inside but the seats are comfortable, with clever flip-down wings on the headrest that make sleeping easier. Seats are slightly wider than Virgin's, pitch is the same. In flight entertainment is on demand however the screen size and picture quality aren't fantastic. Do note though that the cabins and IFE are still better than what Virgin had before they refurbished their Gatwick fleet! If you get a substitute borrowed from Heathrow it's all much more modern and you'll find USB and mains power at your seat. (Rumour has it that these planes will be refurbished in 2017)
Food in my experience is pretty decent and better than Virgin, and all drinks are free. They don't keep coming around with the drinks trolley but if you want something else just go to the galley where you'll also find a big box full of crisps, biscuits, chocolate etc to help yourself to. A plus for me over Virgin is that they carry London Pride bitter as well as lager (Heineken and Tiger).
I've always found BA crew very friendly, proactive and accommodating. If you've got kids you can ask for a SkyFlyers pack.
The amenity kit doesn't include socks or an eye mask but they will provide these free of charge if you ask.
Hand baggage allowance as mentioned above is about the most generous you'll find on transatlantic services. Worth signing up for the Executive Club and collecting Avios.
As the BA 777 is a smaller plane than the Virgin 747 (283 passengers versus 455) you should be off the plane a bit quicker but of course it all depends on whether or not another flight has arrived just before yours - on my last trip we just beat the Virgin Atlantic flight from Gatwick that left around the same time and the people streaming into the immigration hall did not look happy.
Best economy seats are the C and H seats in the second row as there's no seat in front, but unlike exit row seats you won't have people hanging around waiting for the toilet.