No need to go to the US to get a FAA medical, they have AMEs in the UK too. Make sure they can do a class 1, if that is what you want.
AME by Country
If you are deemed to be colour blind, and FAA regs are not as strict on this definition, your FAA medical will be endorsed "not valid for night flight". You will then need to get a SODA, and I think practically you will need to get that in the US. You will still be able to get your FAA PPL and fly day VFR under the FAA regs before you get the SODA. If you want to get your FAA licence, you may as well do it all in the US at the same time.
I am not sure why you would go the FAA route just to get around CAA medical restrictions. If you don't have a US passport/green card you will not be able to work as a commerical pilot in the US, and an FAA medical certificate isn't going to be any use if you want to fly for the airlines in Europe.