Thank you for clarifying where you are at - will make the advice much more appropriate for you. I agree with the previous responses, with the main point being there are 100x the opportunities here. Look at where you would start as a Year 1 regional pilot, compare that to British Airways / Virgin Year 20 Captain. Yes, the UK is shockingly awful in comparison. The caveat to all this is, as I'm sure you're aware but it's always worth reminding ourselves, this is a cyclical industry. What we're getting today could be gone tomorrow. Some sectors are more resilient (e.g. our colleague in brown pants above).
Having said all that, having gone through the EASA CPL(A) to ATPL(A) process, then having got my FAA ATP... I don't think your initial thoughts on getting a UK ATPL in a short timeframe will work. Just to get the theoretical exams complete will take between one and two years before you even start the CPL ME IR and building those required hours. Then to get a job in Europe is much more competitive. So in two years, realistically, you will have a CPL(A) and will be in a difficult market job hunting. To be honest, I don't think you will have started a flying job in two years from now. So as the others have said, if you want to fly in USA, go to USA. You have one written exam instead of over a dozen. It will be so much quicker and cheaper, and you can typically go the Flight Instructing route to get the required hours to get taken on at an airline. If the market is similar to now, you will have opportunities such as Breeze who are paying you to fly the hours from 1000 to 1500.
In summary, come to USA, train here, get the licenses quick and jump on with an opportunity. UK and converting to US does not appear to be a good option. Good luck with it all!
Let us know if you have more questions!