Don't be daft. I never eat fridge cold food and haven't had food poisoning in over 30 years and that time was something that was bought from the work canteen and was, due to time constraints, straight out of the fridge.
Overnight oats eaten a few hours after removal from the fridge will be perfectly safe and a lot more palatable.
And to answer the OPs question, unless it's a literal bucket of them, within the rules to take through security at Bristol Airport. As it happens I took a salad that contained a couple of dollops of hummus through a UK airport last week, that was technically in breach of the 'container no more than 100 ml rule' that still applies at that airport but they didn't spot it. It was a genuine oversight on my part, but I had a nervous minute or two, because I remembered about it just after my tray had gone onto the conveyor to be X-rayed.
As to why I took it with me, and presumably similar for the OP, I don’t want to pay £5+ for something I can make at home in seconds for under £1, that is often nicer than bought food, and for the OP, there isn't a Pret she can use anyway. Not every airport is like Heathrow with a shopping mall after security. Sometimes there's just a WH Smith and a Boots if you're lucky.
Also I had stuff that needed using before I went away and sometimes don't have time to go to the airport shops, we didn't last time, because security took over a hour for no apparent reason (another trip a week before at about the same time of day took 5 minutes).