The tough thing about getting into the US on an immigrant visa (i.e. for a green card) based on employment is that you first have to have the job offer. They don't issue immigrant visas based on your skills and the likelihood of your being able to find a job. You have to have found one already - and then it's the employer who files a petition to have a visa issued for you to enter the country.

There's a fairly good write-up of the process, categories and requirements here: Embassy of the United States Paris, France
It's the Paris Consulate's web page, but I suspect there is a similar webpage on the London Consulate's web page.

The discouraging phrase on an earlier page is the following:
>>In most cases, you can only receive an immigrant visa if someone files an immigrant visa petition on your behalf. You cannot petition for your own immigrant visa.<<

What that means is that, other than for the visa lottery (for which UK nationals are not eligible), you have to have someone in the US - either a family member or an employer - to handle the petition process. And petitioning to get someone a visa is not free. (Last I knew, it costs a couple hundred dollars - more for an employer. Most visa related fees have been increased recently so it could be much more.)

Wish I had better news for you, but immigration is a hassle in most countries these days. Maybe after the elections at the end of this year, things will settle down a bit - but I wouldn't count on it.
Cheers,
Bev