If you get a 1 FAA medical, and get FAA CPL flight licence, when you will come back in UK, you can stay quiete sure, because your medical will be something like a "renewal" of FAA medical & certificate ICAO and will be not a classic initial. So if you are out of limits for a classic 1 initial UK, easily after FAA CPL Flight licence, you will be good fo 1 JAA class.
And even, you will spend less: 264£ rather 400£