What is your final goal? Move to the US forever and establish a flying career or just fly for a few years in the US and then move back to the UK and fly?
If you want a flying career in the UK then go for the UK certificate but if you want a flying career in the US then marry your girlfriend, get a greencard and spend your money on training in the US.
If your end goal is to stay in the US then why subject yourself to the expense and torment of obtaining a UK certificate first? A "frozen" ATPL means nothing in the US...We call that a commercial certificate.
The initial career paths are handled slightly different depending on which side of the pond you are. The fresh UK pilot may fly a big Airbus/Boeing with just a few hundred hours while the FAA pilot will have to fly smaller planes to gain experience first. But with a few hundred hours you can still get a job in the US flying turboprops or business jets and earn more than you would as an airline pilot in the UK.