Well, I am pretty sure itīs writen in several official documents, but for a start and just by googleing "VFR navigation CAA", check this CAA stuff in the VFR guide
http://www.caa.co.uk/docs/64/Using%20GPS.pdf
check what it says on the 3rd line, surprise!!!!!
It says exactly that GPS is not to be used as primary mean for VFR navigation.
You may also have a look in here too to see how the CAA expect you to navigate while VFR
http://www.caa.co.uk/docs/33/ga_srg_09webSSL05.pdf
http://www.caa.co.uk/docs/33/ga_srg_...L25October.pdf
Yes it is ilegal to fly VFR with GPS as primary mean of navigation.
You may like it or not, but it is the way it is.
LEVC