The historic choice has been option 1 because, once you are qualified as an FI, you get to build your hours as an instructor at the station flying clubs. Watch out for the timings, though. You will have only 36 months from your last ATPL theory pass to get your IR issued otherwise you'll have to do all the exams again.
Option 2 would be difficult for me. I don't know how anyone could save at the rate of £10,000 a year. That sort of money usually comes from property.
Option 3 would be a waste of money. If you eventually did an integrated course you'd only get very limited credits and, if you took a modular route and held off the ATPL or CPL groundschool until you left, you wouldn't be able to both instruct and hours build at the same time, you'd have to pay for it.