If on an average day LHR operates at 98.5% capacity, then a day with stronger than average wind will quickly create delays. An extra 10 mph headwind loses 10 miles of available approach spacing or about 3 landigs every hour - something like a 7% reduction in available landing capacity, with stronger winds being even worse. There is no spare capacity to absorb this, so it turns in to large for aircraft waiting to go to LHR and for those holding in the busy London skies wanting to land there. The reduction in arrival spacing can go some way to reducing this delay, but only under strict weather criteria which allow the following pilot to see what is 2.5 nm ahead of him. It is specified that there should be no increase in missed approaches as a result of the use of this procedure, so by implication it is only really used when ther is a bit of a headwind and it is not used to increase declared arrival capacity.
Other airports are also allowed to use the procedure, but it is usually less essential as they tend not to operate at full capacity and most other UK airports use the same runway for arrivals and departures so that inound spacing can be reduced from 6 to 5 miles still allowing a departure to take off in between.
Glad to see someone from the BBC taking an interest and not just making it up like many media types seem to.