Given the N Sea shortages, with a new IR they'd talk to you, and if your face fits probably rip your arm off
With 500 hrs and an IR N. Sea as a P2 would be your best (and possibly only) option if you want to use the IR.
Have you spoken to any of the Big 3 to get a definitive answer as to whether your low hours would be a problem in the current recruitment climate?
Other things to think about.......
Most IR providers are booked some months ahead. You may not be able to get a course before your exams expire
Also worth searching for a recent thread by Helimutt about ATPL upgrade, as following Amendment 6 (as far as I understand it) if you only take the 7 IR exams you can only hold a CPL/IR (ie no multi crew command opportunities). In order to now get an ATPL you'll have to resit the full 13 ATPL subjects (unless the CAA exempt you say VFR comms in lieu of your experience to date).
That said I await a possible flaming from the amendment 6 experts
In so far as costs in keeping the IR valid - an annual IR LPC in a twin (as per LASORs). But consider that if you don't fly procedural IR regularly, passing the aforementioned IR LPC without any practice will be a very very tall order
and so the costs will be higher
Another option is that if you allow the IR to lapse you have 7 years (I believe, albeit check LASORs for the definite answer) to revalidate it before you have to sit those lovely exams again.
Rgds,
FW