I think that, to some extent, his beef with nutritionists started with the McKeith woman. Now, she made a few fundamental errors in her own publicity, not least with giving herself a bit of a false title, and by having such a public profile, she was fair game. Patrick Holford is a self-confessed guru who has a worrying tendency to claim 100% guaranteed success for some things - he too is probably fair game.
BUT - Gillian McKeith brought some very important things to the public consciousness, not least that your poo can tell you things about your health (something which Catherine Collins, spokesperson for the BDA, publicly derided - interesting, as it is a useful tool, frequently used in hospitals to investigate faecal specimens for various indicators of disease!)
Patrick Holford might be an oleaginous self-spun guru but he also has done many people some good, either through his books or through the college he set up. It might not be "provable" in terms of Cartesian principles but if they felt/got better then that surely is a good thing!
Now, the one thing I cannot understand is the position taken by some doctors, and even some dietitians - that changing your diet cannot help prevent diseases and improve your health (outside of a very narrow range of known diet-influenced diseases, such as obesity and CVD). It surely makes sense to the meanest intellect that if you give a system low-grade or the wrong fuel, it is not going to function as well as if you give it premium grade, correct fuel!
What seriously gets my goat though, is that because of these misguided self-publicists, he and others of his ilk have discarded complementary nutrition (i.e. not dietetics)in its entirety as quackery. Stupid, narrowminded and really quite backward.