In my opinion, the (4) B763's will be used for the GLA-SFB & MAN-CPT which are already sold or selling. A third B763 developing & serving STN routes possibly to SFB & others, with a 4th aircraft available for lucrative 3rd Party work and/or an in-house Standby unit.
Once the route network has stabilised, any identifiable 'gaps' in the programme, in addition to scheduled 'downtime', could be filled with responses to demand for extra or reduced capacity by substituting the 763's with 752's and vice versa plus interchanging the 752's with 738's as and when required, as capacity demand dictates.
Once the programme settles down, it wouldn't suprise me if GLA, STN and MAN/LPL each had at least a 763, a 752 & a 738, with a 'spare' 763 & 752 floating between EDI, NCL as well as GLA, STN & MAN/LPL. The bulk of the 73's filling the 'bread and butter' core bases within Scotland & Northern England.
Exciting times ahead.