EP
I dont know of any airline that flight plans 20mins of holding fuel into any london airport.
It's recomended by the CAA, but that falls on deaf ears with oil prices through the roof.
Could you clarify that? It only costs the operator money when it's burnt, not when it's loaded into the aircraft, in reality. For the accountants, strictly speaking, it goes into "stocks".
If it's not used, the next uplift is that much less. It it is used, it was possibly needed to avoid a diversion.
My point being that if an airline does not plan 20 mins hold fuel, it may not be due to the beancounters.
I don't think that this would change much if you take variable fuel prices, additional weight etc into account, although a real expert may rise up to shoot me down about that.