"It only needs one of the two to be on a procedural service for separation to be applied. If the outbound chooses to be on a BS (presumably in order to avoid delays) but there's an inbound on a PS, the outbound will get no benefit because the controler will be applying vertical and/or time/distance separations to keep them clear of the inbound."
Not true NO Seperation is provided between Basic and Procedural and yea it feels very strange sometimes. The notams are only for the specific airport, at my unit if you come off or are heading to the advisory you are procedural anyone else is asked what service they want(which is the system as by CAP774 and from conversation with srg inspectors).
And yea you can carry out an instrument approach on a basic service.