No Hurry!
If Scotland dump the taxes the lowland airports could become hub and spoke gateways for long haul......fare savings potential is enormous....job creation and economic growth significant. Certainly give Paris [4 runways and another 3 at Orly], Amsterdam [5 runways], Brussels [3 runways] some competition.
Common sense dictates that Heathrow is a tactical non-starter.....do you really put all your capacity in one airport.....one incident could close down the majority of UK airport capacity in an instant. Logic dictates that Gatwick is the choice by a long long way.
Unfortunately politics are involved......if you study the Heathrow situation in detail it is evident that the airport runs 10-15% below capacity as a result of operating restrictions [not the "curfew"....the dedicated runway ].
So no immediate hurry......a simple signature on a piece of paper would increase Heathrow capacity overnight.
Is there another reason for capping Heathrow capacity? I really don't know, but if I were writing a novel I might consider that the high value of Heathrow slots would be eroded if capacity increased, and there would be extra competition if extra slots became available.