As MJ points out, Flybe did indeed state a preference for modular applicants to have at least done the CPL and IR at the same school, the reason they gave was that they could see one consistent report or reference from one school covering both courses (not sure whether they were bothered where you did hours building and PPL). However I believe that all low hours recruitment with Flybe for a long time now has been solely pre-selected CPL or MPL cadets and they seem to be concentrating on laying people off these days.
I suppose if you do your CPL at a school you can take time to look at the school, how they go about their business, speak to IR students, aircraft availability etc and if you like what you see then you might as well sign up for their IR (better the devil you know). Conversely if your CPL experience is not positive then look elsewhere for IR.
It is worthwhile doing at least some of your hours building at the school where you want to do your CPL. That way you get used to flying in the local area, you can speak to the CPL students to see what they think of the place and the school can let you know common routes that the CPL examiners use and can give you some structured hours building as preparation for the CPL.
I did all my hours building with the school where I did my CPL and was paired with someone who did all their hours building in the US using a GPS for nav. They really struggled navigating in a new area and with the layout of a new airport and had to spend a fair wedge of cash on extra lessons to get ready for the test.