They don't belong to the regions in an open market, you want them to "belong to the regions" then you need them ring fenced in the regulations.
If you look at "other large hub airports" of a comparable size then look at runway capacity :
LHR 2 vs :
AMS 6
CDG 4
FRA 4
MAD 4
ZRH 3
This is why LHR slots became so scarce, in a purely open and commercial market, the airport has a slot that can be used for
a) flybe Q400 to Newquay (72 seats)
b) Emirates A380 to Dubai (489 seats)
Given the airport uses footfall and throughput to drive commercial retail profits to ensure a revenue stream beyond landing charges, it's a no brainer. The only way LHR gets prop feed on a reliable basis is via ringfencing certain markets and taking them out of the slot pool or finally building runway 3 ( and probably still doing the ring fencing piece). I understand, and correct me if I am wrong, that flybe's LHR operation was a loss maker. Operating Q400s against A320s at a lesser frequency to ABZ/EDI only meant that they had to lead on price. LHR is not the future of flybe IMHO, if there's a future to be had. I suspect there's not, nor should there be given where they ended up. There's a lot of good people being led up the garden path and getting their hopes up I think.