Here is what I recommend, most of which I carry. Some is seasonal (warm clothing in the winter). Most of this fits under the floor panel of my estate car, so no inconvenience or intrusion.
For any eventuality/every day carry
- Mobile phone
- Cash
- UK legal knife
- Mini torch
- Pen
- Lighter
- TRAINING - first aid, RTC scene management (e.g. First Bike on Scene)
Breakdown
- RAC/AA/other breakdown cover
- Small set of basic tools appropriate to your mechanic repair knowledge (no point carrying a multimeter if you don't know what a fuse is
)
- 12v tyre pump (also useful for beachballs)
- Can of temporary tyre repair foam
- Simple spares like screen wash, wiper blades, fuses and bulbs
- Jumper cables or jumper battery pack (not reliant on another vehicle)
- Tow rope (only if you know how to be towed, never let the line go slack!)
- Warning triangle (do not use on motorways and please put it at least 20m or 30m from the car, or before the blind bend!)
- Roll of blue paper towel (more rugged and lasts longer than kitchen roll)
- Bin bag
Accident & Encountering people who have had an accident
- First aid kit appropriate to your skills and knowledge
- Hi viz vests
- Torches
- Fire extinguisher
- Pry bar
- Sturdy gloves
- Seatbelt cutter
- Window hammer
- Safety goggles
- Notebook & pen
- Simple flashing warning lights for night-time
Getting stuck by snow or floods
- Wet weather jackets and trousers (better to stay in the car if you can)
- Warm gloves and hat
- Shovel
- Warm clothing
- Simple snacks which do not need water and have a long shelf life
- 5L water in opaque container to prevent algae growth (8 drops of household bleach in 5L water keeps it sterile and potable over time). Refresh and replace regularly
- If in the winter consider a stove and boil in the bag or freeze dried meals (no need for dishes) and don't forget matches!
- If in the winter consider a foil backed blanket or SOL Bivi bag rather than a sleeping bag. Cheaper and takes up less space
- Drinks pack/brew kit & mug
- Possibly a couple of sturdy plastic bags to go over or in shoes, rather than bulky wellies
Running out of fuel
- Nothing
- You could carry a 5L can of fuel, but this can gel over time and is a fire risk
- You can buy a can at a petrol station
Being stopped by the Police
- A good, calm attitude
- Dashcam
I'm sure BossHogg will have more suggestions
Edited to add that this list is based on my experience as a Casualty Care instructor for Mountain Rescue, RTC management training with MR and First Bike On Scene, a few years in my teens buying/selling cars and working in garages, off-road driving and having been in too many RTCs myself!