Do a SEP IR, it is cheaper?
It is, however you would need to do a conversion to MEP and MEIR at some point, and that would end up being more expensive since most of the IR time is actually in the sim, so SE makes no difference.
You also have to consider the fact that the cost of doing an IR is only ever going to go up, so you have the option of doing it now for say £19k, letting it lapse after a year and renewing when you actually need it for £1k.
Alternatively letting the groundschool lapse, then spending £1k getting the exams back, only to find an IR now costs £22k or whatever so you will lose money in the long run.
If you can find the money now, I would always recommend locking in the IR now if you have any serious hope of an airline position in the future. If there is no hope, then you're just throwing good money after bad anyway and you should cut your losses and move on. Just my 2p.