Tiger_Moth,
I was in a very similar situation to yourself. My airline selection happened during my FI course, and just having got the FI(R), was promptly put in a holding pool for six months. I continued working in a shop and instructing at the weekend. Between August 06 and February 07, I amassed the grand total of 69:20 instructing, including less than seven hours in December!
Doing the rating was great –*you learn an awful lot and it is very different from the whole fATPL route. It's very rewarding. Prior to becoming a pilot, I was involved in training adults to exacting standards, so the FIC was always part of my plan. I just never expected to be in a hold pool so quickly.
The downside to all this is that any school who takes you on as an Instructor knowing you only have a few months of availability will generally only give you trial lessons and the odd one-off student. Luckily, the school who trained me offered a job. Also, it's likely that you won't become Unrestricted: I never got to send anyone off solo, so feel that the rating hasn't been fully used. My airline doesn't allow us to instruct as it would contribute towards our 900 hour limit, and being an FI(R) means that you can't even do private work "on the side". Of course, nothing is wasted, and I could always revalidate and go back to instructing, but life gets in the way of your plans, and it's just not possible at the moment.
There is a time factor as well: it's not just the training for the course, but also finding an examiner who can test you. In this UK, you should allow at least two months for the course and test.
Before you take the plunge and go head-first into a course, consider your motives. If you really want to learn how to impart your passion for flying and train people to be safe and confident pilots, then go for it. As an exercise in keeping flying skills going, it's not hugely useful regarding airline flying; flying an Airbus is very different from flying VFR in a PA28.