Sean, your delay is the amount of time going round the hold. It does not include slowing down early. (how could we work that out accurately anyway?)
As an aside, there's pressure from above about the spacing on final approach. 2.5 miles is being viewed as absolute minimum separation therefore some/many of the Heathrow directors are unhappy providing 2.5 mile spacing when a very slight misjudgement (by us or you) and we'll end up in the office for our equivalent of tea and biscuits.
A change of personnel when the oncoming final director is unhappy doing 2.5 mile spacing and your initial 15 mins could quickly become 22.
(Incidentally 2.5 miles in today's wind takes about 1minute 20 seconds whereas 3 miles with a slight tailwind takes about 1minute 5 seconds.)
Sorry for the thread drift but to get back to your last point, if there's strong headwinds or LVPs forecast then I strongly suggest you expect at least 30 mins holding. I'm surprised it's not company policy with 50kts headwinds.