UK debt can't follow you to the US, and it can't be added to your US Credit Report, they can call you, nothing in US laws to govern out of country collection agencies bothering you sorry to say.
Would suggest changing phone numbers, and going to unlisted status.
Then turn to just written correspondence concerning this debt, keep receipts / copies of money sent to them, maybe money order to them what you can.
They should not be able to garnish US checking / savings accounts, but just in case I would forgo making payments that way.