splitduty,
Your scheduled depature was at 1200, you took off at 1200...
You did fly your flight plan route to the French entry point, got a 450NM direct to the UK entry point, which saved you a bit less than 1 min, got another direct with UK, which saved you another minute.
Then you had to turn once or twice for the sequencing at arrival (here, you lose 2 mins)... and you're just on time to see a departing plane vacate your gate, for him to depart just on time...
This is a part of the big picture.
That's the way flow management works.
With or without CTOT. Your flight is counted, even though not slotted.
If you're too early on an almost overloaded sector, then slot allocations will be implemented for those who are still on the gate. One of them could be the one who is at your gate...
If you're early, you will probably have to wait on arrival. For your gate if you're on the ground, for the runway if you're still airborne.
PS : I reckon there's a +/- rule in every CFMU country. At least, there should be.