Does a standalone EASA Multi-Engine Instrument Rating exist?
This was just published in CAP1855 from the CAA a few days ago, which may answer your question:

Instrument Rating validity, revalidation and renewal for aeroplanes and helicopters
To revalidate, you must hold a valid relevant class or type rating, unless the IR revalidation is combined with the renewal of the relevant class or type rating; no “empty” IR to be endorsed
The UK CAA has deferred this requirement until 10 January 2020 as stated in ORS4 N.1329.
References: Subpart G, FCL.625, FCL.625(A), FCL.625(H))