Eggc,
For a charge to the CAA (about 75 pounds ) you can get an IMC issued onto your licence on the grounds of your IR. This will be valid for 24/25 months from the date of your last IR test.
This will give you IMC priviliges to use while you are instructing. If you renew in the sim then the next renewal will have to be in the aircraft. If an interview does arise then it will only take a day or two to renew the IR in the sim.
Otherwise this still provides you with some instrument capability while allowing you to delay an IR renewal for another year.