The evidence offered by a police officer would be considered, with other factors, by the court. And the civil aspect is perhaps even more important in that any third party claim is very likely to involve consideration of apportionment of negligence, with a loss of control due to inappropriate footwear being a potentially key factor. It is by no means as trivial or clear-cut as you suggest.
This from the AA:
'In 2010, the AA conducted a survey and found that 27% of people said their choice of shoes led to difficulty while driving. Around 5% went as far as to say their footwear caused them to drive dangerously, lose control, or even have an accident. Evidently, the kind of driving shoes you wear really does make a difference – no matter the weather or road conditions.
You may be tempted during warmer months to drive without shoes. However, according to Rule 97 of the Highway Code, drivers should make sure “clothing and footwear do not prevent you using the controls in the correct manner." Failure to do so can result in fines or a penalty. Driving barefoot isn’t illegal, but it’s also not advised.'