I wish people wouldn't say 'there's no harm' or 'mine's fine' the rules are there for a bloody good reason
Anecdote does not equal data.
Infant formula can have bacteria in it. The water needs to be at 70deg to kill the bacteria. A lot of babies have 'upset tummies' or end up in hospital with gastro issues which are generally down to the formula, some babies die. It's at best unpleasant at worst fatal.
The easiest way to do it is to have a 70 deg kettle, but if you don't want to buy a new kettle, boil it and allow it to cool a bit or rapidly cool it to 70 deg, measure about half of the water you need for the whole feed into the bottle & add the powder powder and give it a good shake.
Measure out the additional water you need (do not guess it, especially for very small babies) and then add that to the hot milk feed.
Shake and test.
You will soon get to know what ratio (of hot and cold water) works for you and your baby. It takes no time at all to do and is the safest way to make up a feed.
If you are going out you can make up a feed and take it with you and heat it up in a microwave or cup of hot water, both are fine as long as you give it a very very good shake and test it.
It's OK to make up a few and put them in the fridge if you want to, making them fresh is really no trouble though.
The thing you definitely, definitely do not want to do is mix the powder with cold water, boiled or otherwise, the bacteria is in the Powder not the water. Most people use boiled water, but actually the chances of there being a problem with any mains cold tap water in the UK are virtually nil and the least of your worries.