1. You can gain an FI rating without a CPL but you must have the CPL theory credits (ie do the commercial written exams). To sit these exams there is a minimum hours requirement - those familiar with the JAR system will tell you what it is (I think 150 hours)
2. Many schools take on people to do add-hoc tasks in exchange for flying hours. Just ask around your local area. I take on people to clean the planes, man the desks, sort out our computers, do building work, etc. I would suggest that you are unlikely to get much work at a flying club in IT - it seems that just about every other student is an IT consultant of some kind or other and all of them are willing to exchange their skills for flying. Be prepared to do something more hands-on and you will definitely find something.
3. I have strong views about instructors working without a full CPL. It means you can't be paid for instructing. As you've seen on this site, an instructor working without pay has a tendancy to deflate instructor pay as a whole. Even though I now run a flying club and any cost reduction is well received, I do not like the idea of instructor pay being part of that. Just a personal view - but I think others share it.
4. Instructing is a rewarding career. People will moan about how poor the pay is - indeed it is very low as a PPL instructor, but the pay can become reasonable once you progress, eg CFI, commercial instructing, etc. Don't ever expect to retire a rich man but with a career plan you can survive (just!).
5. Do not see instructing as the fun-packed, thrill-laden escape from normal work. On a bad winter's month (ask any of the people here about december), there can be few more depressing times; a bad batch of students can drive you to despair (and if you think your character is being changed by being in an office, wait until you have the fourth middle-aged know-it-all of the day and you may very well see a new side to your character); and at times even flying in the ciruit for the sixth hour that day can be a little testing.
I think instructing and GA flying is a wonderful way of life and would recommend it to anyone. Just keep in mind that it is a job and like all jobs has its bad moments - and those moments can taste very bitter if you've only taken home £200 in the month for the pleasure.
Good luck with your career and hour building.