My husband was taken by ambulance to be tested on 12th March (negative, just a bad 'flu) but they wouldn't let him bring anything, even a phone charger. I took his medication out of his bag and put it on the doorstep and they did allow him take that but the bag we had packed was left at home.
Fortunately I was allowed pick him up that night as they said he wasn't sick enough to be hospitalised even if it was positive.
My personal go-bag (I've had a few dramatic trips to hospital with children in the last few months) contains a wash-bag with toothbrush/paste, facewash/moisturiser & my medication, pyjamas, clean undies and t-shirt, few snacks (depending on what the children have done to themselves they may need food), bottles of water, kindle, phone charger(s) and charged battery pack.
I can pack it in less time than it takes for an ambulance to arrive or while my husband is on the phone to the GP to ask which hospital a particular incident best suits.