for what it's worth, my opinion would be definitely to get an IMC (the JAR IR is out of most people's price/time range unless you're going for the full ATPL).
it really does open up the flying available to you in the UK, and makes you a lot more confident (subject of course to being current and never taking an instrument flight lightly).
it can either be a "get out of trouble" rating, or if you work at it and stay current, is almost as useful as an IR in the UK . True up north at least where there's not much class A below say FL65. If you're flying a prop single/twin it's not like you're going to be flying into Heathrow regularly...