Hi Kokpit,
In addition to what CZ posted:
The FAA PPL/IR can be converted but at the expense of time and effort. You will have to do the groundschool IR and an additional 15 hours and the CAAFU exam.
FAA PPL/IR holders are also allowed to exercise the same privileges as PPL/IMC holders in UK airspace.
With regards to the issue of keeping your FAA stuff valid. There are plenty of FAA AMEs and CFIs over here to get you sorted out. And you can of course keep the IR valid by ensuring you don't fall foul of the 6 in 6 months rule.
HTH
FD