You at least need to understand why each guideline exists before you break it.
Powder isn't sterile. It can't be sterile. Even if it was sterile when you opened the tin (impossible unfortunately) it stops being sterile at that moment.
You can be reasonably sure that powder bought in the UK is uncontaminated but this is not the same thing as sterile. In fact we're in contact with potential contaminants every day, particularly for e coli which is one of the most common, it's a bacteria which lives normally and healthily in the gut. You probably have some on your hands, in your clothing fibres, along with viruses like flu or the cold virus. In normal contact, this is no problem. Our immune systems can easily handle small amounts of bacteria and so can babies'. Not as much as ours but they can handle miniscule amounts.
The problem comes when something gets into contact with the milk. The dry powder isn't much of an issue because bacteria can't really use it but they can lay dormant. If the powder gets wet, or damp, then you've an issue because given moisture and protein, bacteria can multiply very well.
So your first port of call against bacteria is to wash your hands and use clean utensils when dealing with formula. The next is to prevent any moisture from getting into the tin.
When you make up formula, you can use hot, cold or warn water. Hot is best, because if there are any bacteria present, it should kill them. This isn't 100%, but it will definitely reduce them to safe levels. So it's a good idea to feed your baby immediately or at least put the bottle into the fridge to keep it at under 5C which will cool the bacteria to a point where they multiply only extremely slowly. Therefore you're okay - but not forever. After 24 hours things get a bit iffy.
If you use cold or warm water, you're taking potential bacteria from an unfriendly environment to a great one for them. Moisture and protein. And the closer that environment is to 37C, the happier they'll be and the faster they'll breed. So if you do this and there was a low level of bacteria in the bottle, you'll be fine if you feed immediately. But if you leave it for longer or there was a dangerous amount of bacteria in the milk then you have a problem.