Pete,
The option as I explained last night was:
Get airborne and hold OUTSIDE countrolled airspace at OCK at 2.4A (or in your circuit!) or take the delay on the ground and have a virtual straight in approach (actually it made just a few orbits at OCK)
If I remember correctly, the flight plan was to join CAS on a standard SDR, which is why I guess you got a slot as ALL EGLL inbounds were subject to a delay. That was why, I guess, that CFMU gave you a slot of 1840Z. If you had got airborne at that time, you would then have joined the queue and be given an EAT (which were +30mins of stack entry!)
Seems pointless to get it airborne, burn loads of fuel and put a strain on a busy sector if you can sit on the deck and relax.
Yesterday was also a particular bad day as a lot of the bad weather around that time was between the holds and the airfield. So even though TEAMing was in force (Left and Right for landings) it was physically impossible to get an aircraft 3 miles behind the previous as they were all requesting strange headings just before base leg, resulting larger gaps on final.
In terms of the "slot" bust - well that is to protect the sector of course. As there was only one sector involved, that sector and it's Traffic Manager didn't want the jet in the system too early.
A twr person can help me here, but a LHR landing slot time is an approx time only. It certainly doesn't relate to the current ATC delay.
A trying day for all - and it was slotted in as soon as it was airborne. I can assure you that people were working their butts of trying to reduce the delays, and the LTMA almost ran out of holding areas for the traffic (Some were holding at FL240!)