Water would be fairly important, but I think water purification tablets would do if I had to fill up from a dirty water source (I have a load around the house), but long term drinking from cleaner sources would be OK, and humans can get used to drinking dirty water if we have to. Boiling it all would be nice, but getting a fire going might be impractical in terms of fuel after all the gas cannisters/convenient wood sheds have been emptied.
Food - tins etc would be good, but longer term, hunting would be important, and growing veg, potatoes etc in several locations (so you don't die if a thief finds your crop location) would help. I'm sure there are books about food sources in Great Britain in outdoor shops up for grabs.
Shelter - Warm clothes would be important, luckily I have plenty of them, and a trip to an outdoor shop would fill in any needs. A house would be nice, but either danger from humans or need for food may need temporary accomodation, so I'd bring my 2man tent along.
Weapons - for protection from humans, and also hunting animals. an axe and machete from the garage plus a rifle stolen from my school cadet armoury (plus javelins for reusable hunting!) would suffice.
Transport - I'd try to get a big 4X4 plus spares, plenty of petrol for it (getting it out of a powered down petrol station my be rather difficult though), load that up and camouflage it, cover windows with metal bars - don't want anyone driving off with it when I'm out. Would be important to be able to initially get around quickly to grab supplies, and later to move on in search of better prospects.
Communication - Useful for if there is any coordinated government action, finding 2-way radio bits might be fairly hard, but nicking some from a police station might be an option, but maybe building your own analogue radio may be quicker than searching for another one.. No idea how I'd go about this though - may just have to stick to shouting at people.