Get your breastfeeding support sources in place. Unfortunately the reality is the frontline support (midwives, health visitors, GPs) is very very patchy in the UK and a lot of women get given bad advice which causes problems, or not advised correctly about how to fix totally normal starting-out worries or problems. So it is very useful to know where to turn and who you can trust if you encounter any issues at all or even just have questions.
As for formula when we had to add some in last time, we started with ready made and just moved to powder when we were using enough that it was cost effective. Once you open a tub, you have to use it within 4 weeks and then throw the rest away which is no problem if you're FF full time (I think a tub lasts about a week) but if you're only giving small top ups, the ready made is actually more cost effective as you waste less.
The brands of first infant formula are very highly regulated so there isn't really any benefit to one over another. Actually we just brand swapped all the time last time because (long complicated explanation, in short: Germany) and had no problems doing this.
Then you just need to know how to sterilise/clean a bottle (we did it in the microwave) and how long you can keep made up milk or expressed milk for safely. Write on the bottle/container when you put something in the fridge!! It's really hard to keep track of otherwise and horrible if you end up mixed up between yourself and DP and need to throw away milk because you can't be sure how old it is. Or worse if you realise after you've fed it to your baby! (It happens; they will be fine.)
I would recommend reading about "paced bottle feeding" as well if you want to continue breastfeeding alongside formula or if you're hoping for topping up to be temporary. I found this really helpful and made me feel like I was negating some of the "downsides" of formula that can sometimes make you feel guilty if you really wanted to exclusively breastfeed.