As far as the ATCO is concerned the general point that is important is IFR or VFR as this sets what level of service you can get
That used to be the case pre-2009 - RAS only if IFR - but is no longer the case. CAP 774 states, for all three types of service, that they are "available under IFR or VFR and in any meteorological conditions". ATCOs can of course refuse someone a service based on workload etc but you can't refuse someone a DS because they say they're VFR and you also can't refuse someone a BS just because they say they're IFR.
NS