You can't fly IFR without an IR or IR(R), save with an appropriately qualified instructor in command. To use your IR(R) you need a test.
Unless you have a UK CPL, in which case you *may* have IR(R) privileges - I've been told I do, but have never managed to get a clear answer about what I have to do to be considered current in those privileges (and keep my own IR(R) current, so it's irrelevant to me personally.)
G