Gortex bivi bag if your sleeping under a basha/tarp.
I bought a Heatsheet emergency bivibag the other day and I reckon its good enough to use as a cheap alternative to a more expensive bivi bag provided you sleep on a roll mat and dont puncture it too much.
£15 though so for another £15 you might as well buy the gortex (or equivilent) proper one.
Head torch is a must and a gas cartdridge stove (no need for messy petrol/white spirit on a short haul at sea level)
Boil in the bag food although there are plenty of shops and restaurants etc along the banks.
Water purifier (the choices are endless) although I reckon you could use a millbank bag.