The Monarch slots basically facilitate effecient utilization of 9 based shorthaul aircraft operating 2 round trips a days per unit ( maybe 3 with a night departure on some days).
So there are roughly 9 departure slots between 0700-0900 , 9 arrivals 1300-1500, second wave of 9 departures 1400-1700 arriving back 2100-2300.
There are very few early morning arrivals at LGW and very few mid morning departure slots.
It won't be easy to schedule many long haul services at commercially popular times and
without long turnarounds and sub optimal utilization of aircraft.