Doesn't the sheer volume of flights from MAN to LON (something like 37/day?) suggest that LPL could take a few more without killing it?
BMI would appear to be a more likely candidate, but I'm no expert on slots. The catchment population certainly beats the hell out of INV, even if the propensity to fly might not be so great. (Have bmi considered LHR > DND?).
I don't know how much MAN nearby reduces the need for some connections (e.g. why do LPL > LHR > ORD when you can do MAN > ORD direct), but there must still be plenty of routes where MAN can't really help much. The fact that LPL has no other feeders (EZ must have frozen out KLM & AF) would suggest it must be crying out for something. FR have already got Dublin covered, so the only other major feeder contender would be Lufthansa through Frankfurt. I don't think I'd want to take a bet on the chances of that happening...