To add to HD's point of holding out, when the loading for any one of the inner stacks is excessively high then a/c can be 'stack swapped' to another of the inner stacks and held there instead. I would say that Lambourne is one of the busiest however Ockham sees alot of traffic too. The usage of the stacks can also vary between each day due to the routings into the TMA altering due to en-route winds and weather. For instance Bovingdon will get very busy if the North Atlantic tracks are far north, but then the next day Ockham could be busier with a more southerly flow. As LHR plans to have a good 20 minutes of traffic holding at any one time then I'd expect the answer to your question to be one of their four.