this intended to be used in case of chemical plant gone wrong evacuation, not a zombie apocalypse or teotwaki issue.
As this is not TEOTWAWKI and is in the UK it rules out tents, filters and the like. If there is another Bunsfield there would be facilities set up to house the displaced. Whether a camp bed in a school hall or a room in a cheap hotel, the local authorities have civil contingencies teams set up to care for people in the event of a major incident. In most cases (with the exception of serious flooding or other incidents rendering your home unlivable) you could expect to be back home in a few days.
So in my bag would contain;
Small rucksack (25 litre or so)
In my bag -
Hardcopies of vital paperwork for the whole family & house (e.g. birth certificate, passport, house insurance), laminated or in a waterproof bag
Soft copies of all documents, including the vital paperwork and anything elese that may be useful (e.g. wills, property deeds), as well as copies of all family photos. Also recent photos of house, car, bike, etc (anything that may need to be replaced by the insurance company!) Stored on a hardened USB stick. All documents held in a password protected zip file.
Spare house , car & bike keys
Spare bank/credit cards
In each persons' bag -
Laminated contact details sheet. Listing all immediate family and their contact numbers, as well as numbers and addresses for other family & friends outside the area
Cash (£20 or £30)
Change of clothes
Underwear for 2 or 3 days
Warm/waterproof jacket
Gloves
Hat
Trainers/sandels
Small wash kit
Lightweight towel
Phone charger
Torch
Folding knife
Lightweight/2 season sleeping bag
Personal medication (if required)
Small (minor injury) first aid kit
Spare glasses (if required)
Water (only 1 litre or so to tide you over)
Snacks or eat-from-the-bag meals, sufficient for no more than 1 day
Reading book of choice & other things to keep you entertained while waiting for the toxic cloud to dissipate!
As I would hopefully be going home in a couple of days or crashing with family or friends, this kit is only intended to keep us comfortable for a short period. If it's a local problem forcing me out of my home and there are facilities or family/friends I can rely on I wouldn't bother with a tarp, filter or any other outdoorsy kit.