There is no reason not to try and get a job with the majors but a word of caution having the required hours is not enough. United Airlines have 7000 applications on file so be prepared to work for something les than a major. The best route at the moment is the regionals they seem to be expanding rapidly at the moment, the pay is not great but it is a foot in the door.
The UK ATPL means nothing to a US airline. Luckily it does mean some thing to the FAA who will allow you to convert it to a US ticket. I don't know the details but a good place to start would be the FAA website
http://www.faa.gov
I think that you would have to do some of the exams and probably the ATP checkride, Don't take that as gospel find out for yourself.