t_bell, I am not an expert. I am answering you because you posted and you are clearly upset about being arrested upon returning to the UK after living in America;and I didn't want you hanging near your keyboard worried.
No, you will not be arrested by the custom officials. Many people leave the UK to work or live in America and accumulate debt on their UK credit cards. I have a friend who did exactly that and she was not arrested upon her return to the UK.
What did happen though you may be interested in. She found herself unemployed and living with her mother. She went to clear her debt through bankruptcy and was told because she was out of the UK for over 36 months she could not petition for one. She is in a debt management plan.
There are somethings you can do while waiting for a professional to answer your question. To put your mind at ease contact the British Embassey. Here is the telephone number for the British Embassey in Washington, D.C- 202-5886500. Ask,''Will I be arrested upon my return to the UK because I have credit card debts in the UK?'' Do it anonymously if you are frightened. After making that call, make yourself a cuppa and relax.
Also, US debt collecting laws are different in each state. So look into them in the state you reside regarding your US debt.
An expert will come along who can explain to you the 'living outside the UK for over 36 months and filing bankruptcy rule'. You may not have to do this, but it's good to have the information.
Good luck and please post to tell us how you managed it all.
All I have left is my humour.