It's correct that it was the Working Time Directive that required the UK to add it to UK law in the first place (over the strident objections of the Tory government at the time) but it is the Working Time Regulations Act 1998 that provides this regulation in the UK, and since Brexit the EU Working Time Directive 2003 has no legal force in the UK.
EU directives never had any direct legal force in the UK, or any other member state. The point of the directive is to say "all EU members need national legislation which meets these standards". It's then up to the member states to implement national laws using their own unique systems which meet the requirements of the directive.
As you said, the Working Time Regulations Act 1998 is the UK law implementation of the EU Working Time Directive 2003.
