A rewire would come under BS 7671 and are also governed by Part P of the building regulations, there is a minimum required standard and restrictions on the minimum number of outlets, height and location of accesories and fittings etc, etc. If you are getting quotes from an electrician who is a member of one of the organisations that monitor their members, NICEIC, NAPIT, ELECSA are the 3 main bodies, these quotes are all to the required 'standard', the trouble is that rewires in houses that are being lived in can sometimes be unwanted work and therefore you can end up paying a premium, I assume that you have provided the same detail to all about what you expect in the way of changes to the existing No of sockets, lighting changes etc. Also there is a vast difference in some of the accesories that can be used, perfectly good manufacturers produce Sockets and Switches that meet british standards and others produce the same but the quality may be slightly better, I know of some electricians that supply only MK and these can be upto 4 or 5 times more expensive than say 'click' accessories. Also it is up to you if the work is surface or sunk in to the walls. You could channel the walls and fix the metal back boxes yourself saving paying an electrician a premium to do it. You could also move the furniture, lift carpets and floorboards to save more.
I am currently working on a proicing plan to enable the quotation of such work and I am trying to keep the weighting for rewire of a lived in property to a minimum, at the moment I am looking at adding 5% (to the cost of the same work on new build) but I am not sure trhis is enough.
Tell me how many Sockets and Lights and I will tell you what I come up with.