I second all of the above - especially making sure you have all your photos & documents scanned in and saved digitally - if you can store them on the cloud or away from the house, even better (I have an external hard drive in my locker at work). Not just home insurance, but imagine literally everything in your house has gone - do you remember the ins and outs of all bank accounts, birth/marriage certificates, qualifications, pension info, stocks/bonds, loans, car documents, crochet lovers anonymous memberships? You may need to apply for new cards, certificates etc and if you’ve just lost everything having a list will help you get started rather than being under pressure to remember.
Taking photos of every room in the house is inspired, @IAmInsignificunt ! I keep thinking I should do some sort of inventory but can’t be arsed - photos of each room would be great to start to catalogue where things are.
Op definitely put some clothes in, and a teddy or similar if you have small DC (plus nappies etc if needed; and even a small colouring book and crayons). Potentially you would be stuck barefoot in your PJs (I have an old bra packed, for example, so i’m Not jiggling around)
Do any of your family have prescriptions? Either a couple of days worth of meds would be useful if so; or the numbers of some local 24/7 pharmacies so you can get something if needed urgently until you can arrange a new prescription. An old pair of glasses may be useful as well.
(Top tip - anti bac gel is actually brilliant at getting spot stains off of skin!)
Put grab bag as close to front door as you can - you don’t have time to be scrabbling around in a cupboard for it.
And goes without saying - make sure you have lots of smoke alarms and test them regularly so you have a fighting change of getting out in time to grab anything!