If I were you I'd do it.

I could go on for awhile about the issues with trying to make a flying career in the UK right now, but to sum it up, the opportunities are much better in the USA right now.

As pointed out, you cannot get hired as a DEC in the USA. However, this a good thing, as DECs in the long run are only good for the company. In the USA seniority is everything, it will decide what seat you sit in, what routes you fly, what equipment you fly, your vacation, and problem some things I can't think of right now. In the USA you have much more control over your working conditions compared to European carriers.

Your times are around what the major airlines are looking for, but getting hired at one of these can take some time and your going to face an issue because you are a 'special case', not a bog-standard 121 regional pilot or military pilot.

So what I'd recommend is to figure out where in the USA you might want to live, and get hired at the best (best being subjective to what you consider important) airline that will take you right away. The regionals are hurting and will take you as soon as you get your FAA ATP, but you should also consider places like Spirt, JetBlue, Virgin America, which are sort of half-way in-between regionals and majors. Even these places are somewhat difficult to get on with, so some regional flying to strength your profile will likely be the best option.

Something different from Europe compared to America, your not tied to your job like you are in the UK with 2-3 month notice periods. You can get a job at a regional and if you find something better just give them 2 weeks notice and move your better option.

The green card application is another complicated area, assuming your siblings are USA citizens there is no wait time to file for you. But even then there a couple different ways to do it, each with advantages and pitfalls.