The reason I'm an advocate of the IR-first approach is in part because I didn't do it myself, but wish I had.
I followed the standard PPL then 100 PIC route, got the CPL then did most of my IR training in the SIM. When you look at what some places charge for a SIM+instructor it's comparable with an hour building rate in a cheap single.
It would take some planning, but assuming 35 hours dual for the PPL, 40 for the IR and 15 for the CPL that's 90 hours dual, 10 as a buffer and 100 PIC is still possible.
Let's say you upgrade your C152 to an IFR PA28 for an extra £40 an hour - that's £1600 extra. Add in the cost of 40 hours IR instruction and that's the core cost of your IR if done first.
The points about fewer MEP hours are valid, but it's a cheaper option. You could also choose to do 100 hours of MEP hour building if money is no option. Ultimately it's up to you to find where you feel comfortable on the spectrum.
It's unlikely that you'd have any asymmetric work in a cadet assessment, so MEP experience isn't really too important (you can either fly an ILS or can't) which means that the MCC is really the only thing that's comparable to jet flying, so choose wisely and do it in a relevant type.