An interesting question SK thinks!
I know of a chap who instructed and had no CPL - bloody decent bloke who just loves general aviation. But I'm really not sure of the technicalities - give lasors a try. If it works looks like an excellent hour building route. Remember though that the course is somewhere round £5k so would probably work out evenly with buying 100 hours to get that time P1 in your book. However after your first 100 hours you would be in 'profit'. Added to that you could then be paid for it with a CPL and its looking good.
Only possible drawback may be that you would be bound by the same minima as the guy you're teaching so on a marginal day wouldn't get flying- I think that is outweighed by the school getting you free of charge.
I suspect if you post this on private flying you will get a few people in the know - the guys who instruct for free for fun hang out there
SK