jimbobsmummy - anyone who looks at guidelines as 'gospel' is, in fact, ignoring the guidelines In fact, I'd suggest if the HVs you work with are saying 'no food until 6 mths' for everyone with no flexibility then it's them you need to be asking to do some reading, not people on this thread.
The guidelines allow for individuality. They are less an 'instruction' to individual mothers than a public health statement that mothers should be enabled and supported to breastfeed exclusively until six months, with 'individual dietary needs being accommodated'...that is not the exact wording, but that's the intention, so I paraphrase.
The Cochrane review on which UK guidelines are based did not say much about allergies as they could not find sufficient 'quality' trials to draw any conclusions. It's well-known that the allergy research is unclear and complicated, anyway.
There is some interesting qualitative research on early weaning (you could check out a fairly old paper - published 2001 but talking to mothers earlier than that, of course) - called 'Rattling the Plate'. It is on the web). Mothers introduce solids earlier than guidance for all sorts of reasons, and one of the faults (IMO) of the 2003 guidance is that hardly any attention was paid to supporting HCPs in putting this guidance into practice with their clients.
There is no evidence that babies, on the whole, get any benefit out of solids before about six months and there is some evidence of risk of illness, even in developed countries. Babies who have anything but breastmilk or formula before 17 weeks are at measurable increased risk of ill health.
As an evidence-based public health policy goes, we have it right, I think....but training and support around enacting it is poor.