It might be silly but I don't think it's illegal, at least not according to the RAC.
"There’s a great level of uncertainty surrounding whether or not it’s illegal to drive barefoot in the UK, with numerous reasons cited as to why you’re breaking the law if you do so.
While these may all be valid points, none of them are technically correct. Here is the law in a nutshell: it is not illegal to drive in the UK without shoes on and the same applies for wearing flip flops - there is a catch though.
it is not illegal to drive in the UK without shoes on
You can get behind the wheel of a vehicle barefoot or while wearing flip flops, provided you are able to operate the controls safely. If you do so with wet feet, for example, you might be putting yourself, your passengers and other road users at risk by not being able to drive the car safely. This is illegal.
And while it's not illegal to drive without shoes on, that doesn’t mean it’s right.
According to the Driving Standards Agency – the body that regulates the UK driving test – “suitable shoes are particularly important behind the wheel. We would not recommend driving barefoot because you don’t have the same braking force with bare feet as you do with shoes on.”