I have good news and bad news. You may be in luck because you can open a bank account with an internet bank without physically being in the US - though you may need a US IP address - of this I'm not sure. You may have to scan your personal documents and email them - or - worst case scenario, mail copies. When you are researching, there should be fine print and/or phone numbers you can call. Ally Bank gets great reviews.
The absolute easiest thing is to wait until the next time you are there. Get an account with a biggie like BofA, Citibank or Chase. Pop a few hundred in there. If you need an address - get a PO Box, one that you can pay for every month. Then, get a secured credit card. You know, one of those ones you give them a security deposit and the balance will probably be about $500. Use your new CC to pay automatically for your PO Box. Or if you prefer something tangible that you'll actually use, get a monthly subscription to something - an online magazine you can get on your tablet or phone ($4.99/month) or netflix ($7.99/month and you can use your login in the UK too) or buy something on US itunes every month. Then set your new account to automatically pay off the balance each month. You don't even need to think about it. Then after a year, you'll have proven on time, revolving credit, and by the time you actually move back, you'll be able to walk in to your bank and get a proper credit card that gets you benefits if that's what you fancy.
I have this problem as well - in 2004 I paid off my credit cards after I moved to the UK. Now that I am back and want credit again, I can't get anything other than a department store card. All because I don't have any balances or revolving credit on my credit report; items go off it every 7 years. You may have a wonderful credit score, but without that proven credit history you're back at square one. I'm heading to my bank today to do just what I recommended above.
