I was in the same situation as you and found that as formula was just being used as a top up it was far easier to just get the ready to use bottles! Slightly more expensive but SO much easier and one less thing to worry about! (There will be enough with breast feeding / expressing etc!) x
We use the nuby rapid cool and hot shot method. So you boil water, put 7oz (for example) into the bottle to measure it, tip majority of the boiling water into the nuby rapid cool leaving an oz or two in the bottle. Add formula to the ‘hot shot’ in the bottle, cool the rest in the nuby rapid cool then add it to bottle when it’s ready. The bacteria is killed in the hot shot of boiling water :)
I’ve been using the tommee tippee prep machine since my nearly 5 month old was born, and he’s totally fine.. I am a geek and I measured the temp of the “hot shot” coming out of the machine and it was deffo hot enough to sterilise the formula. When I’m on the go I use the nubby rapid cool and I carry a flask of boiling hot water with me too z
When making formula make sure the water that comes out of the kettle is at least 70C when it's mixed with the formula powder. Water at 70C or more will kill any harmful bacteria. Formula isn’t sterile. When we’ve tried to give our LO formula we boiled the kettle to 70 degrees, mixed it in with formula straight away and put the bottle in cold water for 5 mins - it was then ready, really quickly and we didn’t have to question the safety of this or have to buy products that aren’t approved or recommended by nhs. We brought a kettle that boils to specific temperature so we wouldnt have to wait for it to cool down and could make a bottle safely and pretty instantly.
We have always done the fridge method to save time l, so boil the kettle make up 4 bottles then leave to cool for an hour then put in the fridge and just use up within 24 hours saves so much time and takes the pressure off stressing to do a bottle while little one might be screaming