I'd say we probably have about a 2 month supply, but at the moment that's just things that I currently use anyway for meals such as pasta, rice, tinned toms, lentils, beans, chickpeas etc (we're vegetarian), flour, yeast, peanut butter, jam etc. As it's stuff we use every week then it all gets rotated. I also have things like oil, stock, herbs & spices, tea, coffee, sugar, hot choc, raisins, tinned fruit and custard, tinned soups etc. Again, all stuff we use anyway, so it will get used one way or the other.
Nearer the time I shall stock up on things with a shorter shelf life such as cereals, UHT milk, juice, cereal bars, biscuits, oatcakes etc. I'm aiming to have a 3 month supply of stuff, maybe eaking things out longer if need be, but to be honest, if we're needing more than a 6 month supply then it's time to get out!
I'll also have the freezer stocked with lots of frozen veg because I'm anticipating that it will be fresh fruit & veg and dairy that there will be shortages of. However, be mindful of power supply disruptions etc (especially Northern Ireland going by yesterday's no deal govt notices!). I've got a gas camping hob thingy and some spare gas cylinders for emergency use.
Also think about storing some water. Everyone thinks that won't be affected, but if things truly go tits up, people can't get to work, there's no fuel or power for pumping stations, or the chemicals for water treatment plants can't get delivered, then it's entirely conceivable that there could be problems with the water supply, so it would be prudent to at least have enough for a few days. I don't know where I'm going to store all of this though!
I've thought about having a veg patch or getting some chickens, but I'm thinking that if things are that bad, they'd just get nicked / killed! I keep terrifying myself by reading stories from Argentina after the collapse there. People thought it couldn't happen to them either.
I actually can't believe that we're having to think like this! It's madness!