I know its going slightly off topic:
But according the the draft figures for 2012(latest I found) from the DFT the busiest UK port is Immingham/Grimsby with 60.1 million tonnes. Comparable data from the CAA for 2012 show Heathrow as 1.46 million tonnes. The DFT only shows the top 10 ports with Felixtowe as 7th 26.3 million tonnes and Dover 9th with 22.9 million tonnes.
So I think the figures are incomparable as they completely different types of ports. Do we class Drax power station as a port? Handled/burned approx 7.6 million tonnes in 2012