I think you'll find there are a number of places that would expect to get you through in 3 weeks full time.
RVR800's reference to Harrier/Tornado pilots is a bit misleading; these guys have many, many hours, and have regularly completed procedural IRs in the RAF. Their 3-week course will have included a ME type rating to validate their ATPLs - for some reason the CAA doesn't recognise a Tornado as a public transport multi-engine aircraft
. I had to do the same type rating malarky when I was leaving the RAF, as the C130 doesn't appear on the UK civil register, although we could do our IR on the Herc.
As for part-time work up to the IR, there's no reason why you shouldn't successfully achieve that as long as you work very hard to get the best continuity you can. That would mean trying to get mid-week evening trips in the summer, and as much as possible at the weekends. I doubt it would save you any money, though!