I had a similar issue two years ago, my multi-crew IR on the front of my licence but not current, my IRR was on the back, and I had a current SEP. I could have just done a SE SPA IR renewal and had it signed by an IRE, but that would have cost the IRE fees plus hire of a full airways single which together were prohibitively expensive just to be able to teach IRR and fly under IFR outside of class A air space.
In the end I did an IRR test at a local club in a fairly basic trainer (SRA and localiser only approaches on the test because it had no glide slope receiver or RNAV), and the whole exercise cost me £340 including the CAA charge to move the IRR to the front of the licence. That gives me all the SEP IMC privileges I need, I don’t want to take off or land in less than 1500 metres in an SEP in case the engine stops, nor do I want to fly airways single crew.
The only thing to watch is to ensure that your IR is current when you send the application to the CAA, if it has lapsed by even a day they will move it to the back of your licence when they move the IRR to the front, or at least that was what they told me; as a result I had to wait until my IR was renewed before I could apply for the IRR, but that delay was due to covid so fortunately someone else eventually paid for it.