As far as arrivals are concerned, we (Terminal Control) advise the rate (number per hour) we want and it is CFMU that then allocates the slots to the airlines essentially on a first come first served basis (but see further down).

Arrival flow into LHR has to be thought about quite far in advance as any change in the rate takes around two hours or so to have an effect.

If the rate for arrivals was 44/60 and you needed to slow it down to, say, 36/60 then for the next couple of hours you'd still get 44 and so have to take account of that.

Conversely if you want to increase the arrival rate, again it will take a couple of hours or so before you see the increase really take effect, but in the meantime we can call on any UK or near European flights that are sitting on the ground and ready to go at short notice.

Also as far as arrivals are concerned all the long haul traffic doesn't get caught in flow control at all, as it would be next to impossible to achieve. It is included in the count though so in busy long haul arrival periods domestic and European traffic may get slightly penalised.

Flow control is called Network Management these days and the CFMU has a public area on it's Network Operations Portal. It might be above and beyond the interest of most on here, but some might find it interesting! The 'Tactical' tab is what's happening now, 'Pre-Tactical' tomorrow (or other date set), 'Post Operations' yesterday (or other date set) and 'Strategic' looking further ahead.

Here is the Network brief for UK airspace today. It is produced the day before and gives an overview of what can be expected the following day.