Although the RAC are correct on this topic, be very wary of taking their advice at face value. They speak of "myths" on their website. Well among the most classic of these myths which they maintain is that if you are caught with four defective tyres you can be fined £10,000 (about as likely as winning top prize on the the lottery two weeks in a row) and that you can receive twelve penalty points (completely incorrect).
I keep a pair of unlaced trainers in the foot well for driving, if the weather is good I normally wear tottery heels or high heeled boots (I am only 5' tall) neither of which are good for driving. I have occasinally wandered into petrol station offices in the trainers, not a good look.
Well, I've seen a number of ****s (people) riding motorcycles with flip-flops, & shorts! Those will take care of themselves.
In a car bare feet & flip-flops could cause loss of control & injure someone else.
Remember, it's not all about you.