Mr Fogg - a 100KVA DG would just about fit on the back of a pickup. Hardly a truck - unless you're using red-neck terminology.
The total standby load of a place like LGW will not be as great as you might think. Many non-essential systems can be shutdown. BMS can automatically load shed. Yes you might still need a few MW but a 3MW diesel set will just about fit in a 40' container. Parallel up a few in an auto-start, auto-sync config and you'd be there.
My guess is that LGW does already have backup power systems but that they were also affected by what took out the mains - i.e. flooding of substations. Water & Electricity do not mix well. Ideally you'd keep main and stand-by in separate locations (not just to avoid flooding - bomb attacks are another possibility) but it's not always feasible to totally segregate the systems.