IMC !
Regardless of anything else, it'll add a lot to your "get out of gaol" options if things are going wrong, anywhere in the world. I really enjoy the challenge of accurate instrument flying, which is the biggest "new" thing I learned - when I got to it - in a long time.
And if you do it now, you'll keep it, and as has already been said, it's likely to grandfather into some future European IR-lite qualification.
Re: ME - why? I've seen little evidence to suggest that flying multi-engine adds to your enjoyment, or even your safety much, but I've seen much evidence that it'll add a lot to the bills.
And have you done tailwheel? It sharpens your handling, and opens up lots of interesting (and often cheaper) aeroplanes.
But anybody without one (or an IR), with any all-weather aspirations to fly in British airspace, should be doing an IMC at the moment, if they can afford it, in my opinion.
G