I find it interesting that Northolt has, yet again, raised its head. There are many advantages, most of which have already been described. Similarly, there are a number of expensive problems, not least the fact that the airfield would have to meet CAP168 criteria. However, this is probably far cheaper that knocking down the Holiday Inn! Flow rates could be a problem because, as already stated, Northolt is almost a satellite of Heathrow and all the aircraft use the same stacks and similar SIDs.
The airfield certainly has the capacity and, although only 5500 ft long, it is plenty big enough for BBJs etc to go accross the Pond.