I loved doing my IMC rating (well IR(r)). I have used it extensively over the last couple of years, it has made me much more confident of my own ability as well as given me many more options, especially for longer flights.
To have the option to depart from the UK in solid IMC, pop out on top before reaching the border to the continent, and fly VFR on top to your VFR destination on the continent suddenly greatly increases your options.
Remember however that you won't be able to fly "in any weather" there are certain factors that may sway your go / no-go. Such as fog, CB, thunderstorms, lightning, strong winds... etc... But otherwise it definitely opens up your options!
With regards to Elstree, I have read and heard of people using Northolt's ILS to break the cloud-base, and if they have sufficient height & visibility breaking off to go back to Elstree low level. Seems much closer than Southend! Even if Southend does have a train station right there.
As you can now also do the CB-IR course, some if not all the hours during your training are transferable for the full IR rating I think its even more relevant to do!!
Hope this helps!
Alex