£3k will about get you a JAA PPL(A) in the US, double that for the UK.
An ICAO compliant PPL, means one that meets certain international standards. This includes the JAA one, the old UK one, the FAA(US) licence, South African, etc. Those nice chaps at UK-CAA will allow you to fly a British registered light aircraft on a non-JAA / non-UK ICAO licence so long as you stay with it's limitations, and inside UK airspace. But, to upgrade to a commercial licence, you'll need the JAA licence anyway, so you may as well start with that.
Costwise it gets worse. You've spent £4k getting a JAA PPL (£3k doing it in the USA, then another £1k to learn how to fly again in British airspace). To fly commercially, you then need another 150+ hours (costing an absolute minimum of another £8k), about another £8k to do a CPL course, then....
£20k+ in debt, you have to fight it out amongst everybody else, who almost certainly has more experience than you, so the only way to get work is probably as a flying instructor, which means another £5k for the course.
PLUS if you were medically discharged from the Navy, you may not get the class 1 medical certificate needed to be paid to fly anyway.
In what appears to be your position, combined with a deep desire to fly for a living, I'd suggest:-
£3k for a microlight licence
Another £5k or so to buy a cheap aeroplane and hoursbuild.
Another £3k for a flying instructor course.
Then, about £11k later, you are in a position to work as a microlight instructor. The money is liveable on, and the work is guaranteed - there is a big shortage of instructors all round the country.
Or possibly look at hang-gliders, paragliders, balloons - where similar costs and medical requirements will apply.
Good luck,
G