Chances are from what you say that the MIG policy was paid out by RSA to B&W. There should be a breakdown of the claim in the papers they have supplied to you, or you should ask for one if not. Rather than ignore their letters you should follow the advice on this site and put them to strict proof of claim. If they ignore your requests for information or explanation of mis-selling then simply repeat the request in a polite but to the point letter and lob this back to them.(posted 7993 days ago)Eventually they will either have to produce the information or run the risk of looking obstructive under the court procedure rules.
From what you have said I can't see that they would risk a court case so their plan will be to grind you down in the hope that you will eventually cough up some money.
If you are not intending to return to the UK before the 12 year period as defined in the 1980 limitations act is up then my advice would be to refuse to acknowledge the debt they are claiming and keep asking for information until you have obtained all the records.
If you are intending to come back to the UK and live then you might want to clear your credit record in order to purchase property or other reasons. It would pay you to reach a deal from the US since they are most unlikely to try to chase you through US courts, and you can negotiate from a position of strength.