Highwide this lets make Heathrow 24 hour mantra and that would be a simple solution is an utter red herring.
These days no one would want or choose to depart to anywhere at 3.30 am period it just ain’t happening !
It’s no longer 1973 and flying on some no mark charter to Palma or Rimini . Passenger aspirations have radically changed
Indeed few want or choose to depart at 06.30 right now!
However you do realise contrary to popular Anet and Dryed fruit myth Heathrow is already 24 hour although heavily slot constrained between 11.00 and 06.00 which as a local resident is beneficial.
There are actually around 18 slots available mostly used and available before 6am each night used by early long haul arrivals.
I will tell you the Boston/Hong Kong depending on season is the first and what ever operates it I certainly here it and it ain’t quiet at 04.40 !
Given the UK geography, long haul and inter European travel preferences frankly don’t lead to demands for departures after 11.00 or arrivals much before the current 04.40 time frame
UK isn’t the Middle East where the time frame for those transits do mean stupid o’clock operations.
We are on the edge of the Eastern Atlantic Heathrow is the largest US landing point in Europe.
Major American airports are also night restricted and those late evening departures Boston excepted always will arrive around 6 am and after.
From Asia it’s much the same for overnight flight although many from this area arrive in Europe late afternoon.
The only potential beneficiaries of any slots in the current heavily restricted time frame could be a few freighters, however boxes couldn’t care less where they enter in country - Stansted provides more than sufficient capacity for those today .