Whether a sump is required depends on the radon risk at your location. Technical Guidance Document C requires additional protection, including provision for a sump, in designated High Radon Areas, while elsewhere a properly installed radon barrier is generally the baseline requirement. Your designer should confirm the requirement from the EPA radon map.
The requirement flows from the radon risk category of the site. Where the EPA map identifies the area as a High Radon Area, the guidance calls for a radon barrier plus provision for active sub-floor depressurisation, normally a standby sump. In lower risk areas a barrier alone is the baseline.
Even where not strictly required, many designers include a standby sump because the additional cost during construction is small compared with retrofitting after occupation, and it provides an immediate remedy if a post-completion radon test comes back high.