Here's the situation:

Once upon a time a young man (late teens) in the UK got involved with a group of pseudo-friends in committing a series of crimes.

He eventually testified against his co-conspirators and did little or no prison time himself. But he was at risk for retaliation, so he wanted to disappear.

He didn't have much honest job history at that point, but he did have some technical skills and physical fearlessness.

Eleven years later he's living in the US and working in construction/as an electrician.

How did he get there?

I've tried researching the relevant bureaucracies online, but they don't address enough specifics that would apply to this situation.

Would the British authorities have been any help in terms of witness protection, even just giving him a passport with a new name and telling him he was on his own?

Could he have gotten a US work visa before emigrating and later a green card? Or just show up illegally and fake the paperwork somehow?

Would it be more or less plausible for him to have spent a few years someplace like Ireland or Australia in between? Or working on a ship?

Could this whole reinvention have been done completely legally, and if so, what authorities would know who he used to be? If not, at what point would it be most likely for forged documents to be necessary, which he could use to qualify for legitimate ones on false pretenses?