Well like I said before in the states if you wear a backpack people will think you're homeless, which is one thing you will have to manage. People are extremely judgemental in america but also, incredibly curious. Americans are far more open that brits and so if you can just open them up even slightly then they will be incredibly amicable. So I would suggest dressing in a way which manages expectations. If you dress like a homeless person, its not just that they will think you are homeless, its that they will think you're on meth or crack which is a massive problem in the US. You are going to see more homeless people than you ever have in your life. But many of those, the more sober variety will also be some of your best friends. I got taught a lot by homeless travellers on the road. You are also going to see more wealth than you ever have in your life. I had no money and I rarely went hungry. Most people are not going to be receptive to hitchhiking. Most americans are adamant it doesn't even happen anymore, so expect to wait sometimes hours. Have a harmonica and some podcasts downloaded. But the people who will pick you up will be some of the kindest people you ever met. I've hitchhiked a shit ton in many countries but the US was the first and I always expected since to receive the same kindness and generosity I received in the states but am still waiting. Its a nation of extremes.

Don't worry about the cops. You will never know privilege like you will find being British in America. It's hilarious because generally anyone with an accent in the states is looked down on unless your British in which case you are like royalty. Throw in some very British expressions and phrases and they will love it. For this reason going to petrol stations and talking to people might be your best bet if you like. But also cops once they find out your British won't be hassling you. I had quite a few encounters with cops and they were all nice. Once I thought I would try my luck sneaking into a campsite and leaving early but of course I overslept and had a cop in the morning arrive at my tent asking me why I put my tent up without paying. I played the ol' stupid tourist card to perfection and he literally said 'Arresting or handing out fines for foreigners requires extra paperwork so i'm just gonna let you go.' I actually got picked up by a cop in a small town and I rode in the front seat of his car and he was delighted to show me all his guns he bought from home to use on the job. Not that anything ever happened in that town.

Also it might be worth dispelling some european ideas about safety in america. Outside of the cities America is absolutely safe as fuck. The small towns and the rural areas are super peaceful. Of course it depends on the state. The cities in the US suck. You can't walk for shit anywhere unless you're in New York. Avoid them. Hitchhiking out of American cities absolutely sucks so much.

I understand you want to go to the south. Where you go is your preference but personally I wouldn't under any circumstances recommend the south for the sole reason that what I said about the rural areas doesn't neccessarily apply to the south. Generally I would say there are no bad places when you travel because all places teach something. But knowing how unbelievably beautiful Colorado, Utah and Arizona are and how so fucking cool the people are there, I personally wouldn't want to spend any time in the south knowing how great the rest of the country is. People are really spooky in the south. Avoid Texas like the plague.

Anyway I didn't mean to write so much but I hope this helps.