Further to Stik's excellent reply, I can reveal a little more of the origin of the Jet.
Gatwick, Heathrow, and Manchester are all connected to the UKOP (UK Oil Pipeline) system. This pumps Jet & other products produced from Esso Fawley, Texaco Pembroke, Total/Fina/Elf Milford Haven, Shell Stanlow,Lindsey & Conoco Immingham, Phillips Billingham, and BP Grangemouth refineries.
However the UK also imports a lot of Jet A-1 into BP's terminals at Isle of Grain & Hamble, Atlantic Power at Immingham, and Serco Avonmouth Terminals. These terminals also connect into UKOP.
There are also numerous smaller Jet storage terminals around the country, supplied by sea or rail. These generally supply regional airports by road tanker.
I spend about 30% of my time in work investigating seabourne contaminations of jet fuel- you'd be surprised what goes on out there!