I'm not sure why advice changed, but the UK's advice is quite overkill compared to other nations. Most recommend sterilising for a reduced time, just for premature babies or not at all. Most don't advise boiling the water or using hot water. It's interesting. I don't think it's a conspiracy to put people off, but I do think it's from a base of needing evidence based advice via NHS.
I would guess that around 2000 there was a study which showed increased risk from making up in advance, or perhaps that people were fudging it and using one from a day and a half, so they changed the focus to make up fresh. In practice, make up fresh is extremely inconvenient, particularly when combined with advice to feed on demand (you have to wonder who decided to release both of these pieces of advice, I would guess the latter came after 2000 though, routine was king then, less popular by the end of the 00s) - and many people were receiving the "make up fresh" advice over and above the "use hot water" advice, and hence making up fresh with cooled boiled water. So in practice, advice which pushed "make up fresh" has resulted in more babies receiving less safely prepared formula. Because (as I understand it) the hierarchy goes:
Ready made UHT
Made up fresh with hot, flash cooled to baby temperature
Made up with hot, flash cooled, stored no more than 24 hours
Made up with cooled boiled as needed
(It would be interesting BTW if a study was done comparing bacteria levels, both with normal formula and deliberately contaminated formula.)
And now you have the Perfect Prep method which had been touted on forums for years but is now sold as a machine, without the shaking step between hot/cold the forum advice version usually has, and we don't know quite where that ends up in the hierarchy.
But really every level of it is safeguards against the others not being performed properly. The UK guidelines are designed that if you skip or fail at one or even two steps, the rest should ensure you're OK. Perhaps this is because we have such a high formula feeding rate compared to other nations and more of our very young babies are formula fed, whereas in other nations older babies on average are receiving formula and their immune systems are more robust?
- Wash bottles well in hot soapy water (remove traces of old milk, bacteria)
- Sterilise as well (in case any remnants left behind)
- Wash hands before touching bottles, ensure clean surface to prepare bottles, don't put the scoop down on the counter (avoid introducing new contaminants)
- Make up with over 70c water (kill bacteria present in powder)
- Boil water (kill bacteria in water)
- Don't keep bottles hanging around at room temp (prevent small numbers of bacteria multiplying)
- Cool quickly, store in fridge (reduce time in "danger zone")
- Discard after 1 hour after saliva introduced (more multiplying danger)
- Discard unused powder after 4 weeks (combination contaminants and reduced quality)
Certainly I had always felt these were extremely important but in actual fact when DS2 was quite reliant on formula I was forced to admit that it's extremely difficult to meet every condition perfectly. When we first switched to powder I was following DH around snapping that he had just touched his jeans after washing his hands and the counter wasn't clean enough (the counter, of course, was a bloody state because nobody keeps their kitchen spotless when you have a newborn baby) and had he levelled the scoop exactly and then I felt he didn't cool the bottles correctly - he used to make them up, and we did use boiling despite local recommendations stating water which has cooled to 40-50C - and then stick them in the fridge door. He had to tell me to back off in the end and point out that even really stupid people have babies and formula feed and the instructions have to have a bit of room for error. DS2 was fine obviously.