Not going to comment on the ethics of doing this or pass any judgements. In general, bad credit does not follow you on an overseas move -- the credit reporting systems for the US and UK work differently, despite in some instances being part of the same global corporation.
For the US, bad credit will eventually work itself off your credit report around about the 7 year mark. It may be longer than that once you factor in the time it takes for the account to charge off, be referred to a collection agency, etc - the collection agency continues to chase it, extending that 7 years out some. Bankruptcy stays on a US credit report for 10 years - or in some scenarios -- potentially forever when it comes to applying for large insurance policies or working in certain highly paid financial services-type professions.
There is the odd creditor who may chase you for a debt internationally -- I have heard of GE Capital, for instance, referring UK debts to a US collection agency if they find out the debtor has moved to the US. But this rare.
Being in debt (in & of itself) is not a criminal offense, in the US or the UK - you aren't going to go to jail or be arrested for it. The days of the workhouse are long gone - thank goodness! There are exceptions if the creditor can prove you committed fraud but this is very rare & very difficult for a lender to prove. You can go to prison in the UK if you don't pay things like council tax, court fines, etc but again this is very rare & willful negligence in paying the debt would need to be involved.
But if you've ever planning a move back - think very carefully about walking away from a debt, because a bad debt is going to stay on your credit report for a good long time!
The advisable thing to do is try to work something out with the creditor involved, or consult a third party debt advice counselling service. There are some excellent services like that out there - in both the US and the UK.
