(milkmummy - better to start a new thread on this. It's a good question)
Daisy - the evidence on allergies being reduced if you introduce solids at 6 mths as opposed to 4 mths is thin - and as has been said, we are not really in a position to glean definitive data. Certainly, solid food before 4 mths has been shown in many studies to have a connection with allergy/atopy/serious metabolic conditions.
There's a common sense view that not being in a hurry to intro solid food until the baby is capable of holding, licking, tasting (ie until he does not actually need purees) follows what the baby's own 'system' is geared to cope with - anyone can get purees down a young baby just by shovelling it in, and the baby is not really in control. Babies are not usually capable of holding, licking and tasting new foods until - guess what? - somewhere around 6 mths. Very few babies of 4 mths show interest in doing this. This is a powerful clue to when babies reach this stage of development.
There are no papers which reveal any benefits to solids before 6 mths (except in individual, special cases where an individual baby might need special treatment) and no papers which show harm if solids are not given until then. The good evidence is on infections, as you say - but to me, that's quite a powerful bit of evidence. Sure, in the UK babies are not going to die of an infection, but even a mild tummy bug or an ear or chest infection is miserable (from memory, these are the infections cited in the research). It means a trip to the doc, possibly medication, hassle, pain.
The PubMed listed study you quote: you have misinterpreted the conclusion. It says there is no evidence that solids after 4-6 mths, ie after 6 mths is beneficial.
There is no magic date for all babies. But when it comes to policies,it makes sense for parents to know there is no harm in a milk-only diet for 6 mths for most babies, and considerable evidence of benefits. In addition, they should be encouraged to offer their babies ordinary food, for the baby to explore and discover. This makes developmental sense, too, as the baby's skills in understanding the link between his eyes, his tastebuds, his fingers and hands and his mouth, are all supported, in a way which can't be replicated by spooning in a puree which he doesn't really see and doesn't handle.