As I understand it (based on what I read in their house magazine) all orders are processed through the Interflora cart and channelled through the Rose system but I believe (and this would need an IF florist to confirm) florists have to have all the designs Interflora sell including the drop ship items. It says that local orders go to the florist involved, not on the rotational system, and that the data belongs to the members and cannot be used by Interflora. The sites cost £39 a month.
That said I would always recommend a florist has their own site. Because while it may seem easy and relatively economical to use a supplied site, I personally think any online presence should be totally independent of a wire service and with e-commerce becoming more affordable, is within the grasp of most retailers.