The NHS publications reference WHO guidelines which do recommend waiting until "the middle of a baby's first year" (interpretation of that could be interesting in itself) but any awareness that the WHO guidelines have to give the safest option for all babies (some of whom are growing up in unsanitary conditions in the third world where access to clean water etc is very limited) is sorely lacking. The WHO also recommend breastfeeding for 2 years - again, having looked at the research, the major factor behind this recommendation seems to be that breast milk is the safest drink in some areas of the world.
As the evidence based parenting guide "Bumpology" says - the current NHS advice is to wait until 6 months but there is no evidence to suggest that weaning between 4 and 6 months is harmful to your baby. So, now we are beyond 4 months if Bumble's dad and I decide to offer him (suitable) solids at least I feel confident that we won't be setting him up for long term health issues based on current research!
One anecdotal aside - I am yet to meet a single parent who did wait until 6 months to start weaning...