I have read somewhere about "18 slot pairs", presumably involving 9 based aircraft.

I'm thinking BA longhaul, maybe a few bits and pieces to Level and Iberia / Vueling?

Can't decide if I should put Aer Lingus to Eire either high up on the list or nowhere on it at all.

I can see Vueling. They applied for a 2 aircraft base at MAN but I can see all resources shifted to Gatwick to run core Spain and Portugal routes.

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.

BUT. BA / IAG would be free to shuffle around slots to accommodate anything they’d want. An early AM departure could turn into an arrival ?

Shuffling doesn't increase the number of early arrivals ,unless they dump current BA shorthaul nightstops which compromises those schedules and affects utilization of those aircraft.

Would they be allowed back on the BFS/BHD route since FR gave up the route in favour of STN. All part of the EU conditions of BA buying EI they had to give up the BHD LGW flights.

FR have to hand the slots back to IAG ,so unless someone else wants to pick up the BHD/LGW route and use these slots ,I don't think there is anything to stop either EI or BA reintroducing the service with these slots ( regardless of the OM slots ).

Think IAG are concentrating on yield improvement on BHD/LHR ( hence frequency reduction this winter) , so they probably don't want to add London capacity elsewhere unless they think it's totally different market.

As I suspected

Monarch had 154 slot pairs (not total slots) a week at Gatwick which is equivalent to about 22 departures/arrivals a day. So room for quite a bit of expansion. On some days in Summer departures were higher than 22, more like your 25 but less of course in Winter. I think Monarch based 10 aircraft at Gatwick during the summer each operating 2 to 3 departures a day (so 2 or 3 slot pairs).
With greater flexibility of scheduling as a result of these new slots BA could put together an impressive long medium and short haul schedule. I expect planning has been going on longer than we realise. I remember when Easyjet bought the Flybe slots BA was interested but said they had come about too early for their plans. With BA Gatwick now in much better shape and I believe profitable it is now time to put plans into action. However I expect other IAG airlines may get a look in. I am not convinced about Level turning up that quickly or at least substantially. The refurbished, densified and fully paid for BA 777s will allow them to compete more effectively with Norwegian. The BA brand is strong at Gatwick and certainly long haul lucrative. I doubt if they would wish to dilute that. I also expect some Airbus aircraft will be transferred from Heathrow or leased in to expand short haul network. New A320/A321 are due at Heathrow so others can be kept longer if necessary. I am also not convinced about Vueling/Iberia Express expanding greatly. BA is popular and known at Gatwick. Again why undermine it? That is not to say that other IAG airlines will not expand their presence.

But there are plenty of early morning arrival slots available at Gatwick. LGW is only severely slot restricted at peak times for based aircraft (early morning departures, late evening arrivals).

If BA want to use the slots to expand long haul, the biggest problem they will face will be gate space.

Gatwick extending Pier 6 and plans to move A380 to Pier 5.

Gatwick Media Centre ? Gatwick and Bechtel partner to deliver major construction

NAX are announcing the purchase of 28 LGW slots for Summer 2018 from LLC.

Excuse my ignorance, but what's LLC?

Sorry....
LLC/Small Planet Airlines of Lithuania.

In Oban

Yes it's a lot easier for everyone to understand when the airlines or airports official IATA codes are used.

Apologies... as viewing my post again I see I should have stated it is 28 slots PER WEEK purchased.

Not for me. As an ex ATCO I’m much more familiar with the ICAO codes, both for airlines & aerodromes.

Were Small Planet Airlines using the slots, ie what routes/rotations are being dropped?

Yes kcockayne but you will find no condescension from that quarter I believe.