Take that with a pink of salt though. They SHOULD be those things...but sometimes they are moody/rude/difficult. This isn't UK specific as I've heard negative things from loads of different customers agents. It's unlikely they will give you a hard time as you have a visa (they might ask questions - and the polite and happy ones will ask you things they're genuinely curious of like "are you excited" etc.), but they might not be the most friendly or polite so don't assume there's something wrong if they are anything less than pleasant and polite.
US immigration officers can be very scary at times.
One time when we were going through US immigration in Toronto before boarding a flight to the US, my dad had forgotten to sign the bottom of his landing card... the immigration officer practically threw it in his face and shouted at him to go to the back of the 2-hour queue! Luckily we were travelling with my grandmother and had wheelchair assistance so the porter took us through the crew line, as our flight was due to leave a few minutes later!
Also, when I first entered the US on my student visa in 2003, I had accidentally packed my important DS-2019 immigration form in my suitcase instead of my carry-on. I was detained for 2 hours in the back room and then escorted to my case to retrieve it. One poor kid in that back room needed the bathroom and asked where it was. He went down the corridor, then came back a few moments later saying he couldn't find it. The immigration officer shouted at him, telling him he'd had his chance to go and told him to sit back down.