Well, whilst everyone is correct in their information so far, it seems that the original post was looking for a mixer/shower tap as opposed to a mixer shower- which is seperate to the tap. I stand by my original posting at the begining, if this is the case. Also just because a system doesnt owrk with 5ft of head in one location, it doesn't mean that it will not work on another. It is to do with the differential betweem the cold pressure (mains) and hot water (gravity). My experience is that it will be unlikely, but not impossible for this setup to work satisfactorily.
A professional would fit a PRV as standard, but this is a DIY group where you can add components on a trial and error basis and save some money if you dont need the "full" setup. A proffessionals time would cost more than the component in question, but in this instance (given this group) this is not the case.
No harm will be done in this instance by trying before buying a PRV.
Calum Sabey (Newark Traditional Kitchens 01556 690544)