A third way?
The third way I propose to you is to consider doing a Single engine IR as a follow on/combined with an IMC (if you have an old CAA PPL you can even get a 12 hour reduction off the course up to mid next year).
The single engine IR is cheaper and easier to do and you will definitely use it more as it gives you access to airways and a deeper practicality level than IMC. If you go the single engined instructor way of building hours it helps teaching IMC.
The single engine IR holds your JAR APTL credits, it is cheaper to renew and if you want to unrestrict it to a twin engine IR, the minimum hours required to upgrade to twin IR (according to JAR) is 5 hours dual + exam (if you have a twin class rating).
The route I'm proposing to you will work out more expensive in the long run than the short sharp one year route. This route is best suited to those people who want to take a more leasurely 5 year+ approach or maybe just want to do flying as a part time vocation and have a real life at the same time.
This route is a half way house and it allows you to slowly build and gain more experience with less worry on the finance and pressure on the CAA deadlines. You will only have to commit to that twin IR test at the very last minute and you will only have to do a minimum 5 hours to get it - that's nearly the same as a twin renewal!!!
I'm sure the experts on the JAR matters will confirm what I'm saying is correct.
Best regards and good luck with your training what ever route you take.