Just read the operator guidelines and compliance guidelines. They appear to be obliged to fully investigate all complaints and resolve them within 6 months and to keep full records for a further 6 months after that date.

I suspect the police and regulators' interpretation is that they (Uber) cannot possibly investigate sexual offences and so the only correct handling of such a complaint is to immediately notify the police.

But you are correct... the phrasing is ambiguous in the guidelines. But then... UK law :) everything is ambiguous and the default is whatever a "reasonable person" (typically a judge and jury, but in this case the regulator themselves) would interpret the guidelines as being.