Well there are always 24 hour Tescos Emma :) It's not like it's intended to make it impossible or forbidden, it's just supposed to remove it as a first run-to, to give you the time and space to try other things first. It's more of a psychological thing - that if you're expecting to fail you're more likely to if that makes sense, and some of BF success is related to confidence. An article I read years ago likened having formula and bottles "just in case" to a young man on his wedding night having a dildo on the nightstand "just in case" he couldn't perform for his new bride - somewhat intimidating in the face of something that people can sometimes feel very anxious and fragile about. To be honest I feel like this advice is mostly relevant for people who have partners or family who are very jumpy about breastfeeding and who are likely to be hovering and questioning constantly and asking to give a bottle when the baby is absolutely fine.
Some people feel that's too extreme and would prefer to have formula available immediately because it lessens the anxiety and pressure of being literally the only option available. I think it's really a personal choice, I don't think anybody should be saying "Ooh no you mustn't have formula" OR "You've got to have some - just in case". It's really down to what makes the individual feel more comfortable. Formula as a supplement can be harmless or it can seriously undermine breastfeeding and it largely depends on how you use it. If you are turning to formula to solve what you (or other people, sometimes) perceive as problems with breastfeeding then you're likely to end up with more and more problems, because formula cannot solve BF issues, and serious issues do not tend to go away magically in time. If you're using it as part of a planned approach (especially if you understand how BF supply and demand works in the early days, e.g. not leaving frequent massive gaps, breasts not needing time to "refill"), or sporadically to give yourself a break every now and again or as an emergency sticking plaster because BF is not working/you're too tired or sore and you just need the baby to be fed, then it can be absolutely fine, but in the latter situation you do need to seek out support for your BF issues as well if you want to continue (preferably after you've had some sleep, or a break, or a double gin and tonic or whatever it is you need). It's not going to harm BF in those situations so having it in the house or using it before X magical date absolutely isn't a death knoll for BF or anything like that.
My opinion is more - for me having the formula as a backup is unnecessary - we'll muddle through if need be and it's only ever a few hours until the shops open, even though there aren't any 24 hour shops where we are. I'm more anti buying things we don't need than anti formula. We can get it later if and when we want to use it, which I think is likely to be yes at some point, I can't be arsed with pumping this time.
After birth - they come out pretty satiated having had constant nutrition in the womb so there is no immediate rush but most babies will latch spontaneously within the first hour if left alone. Google "The breast crawl" for some amazing videos of this. DS did not (not that we did the proper positioning for the crawl thing, I didn't know about it) and once we were taken to the ward we both promptly fell asleep for about six hours and I woke up in a panic going OMG I haven't fed him he's going to starve why did nobody wake me up?? A lovely midwife calmed me down and showed me he was totally fine and helped me get him latched and it didn't seem to do him any harm. Nobody is going to let your baby starve so don't worry about this. If you can't produce any breastmilk at first or you can't get a latch at all then they will help you to express if you want to and if you still can't/don't want to then they will absolutely provide formula, they wouldn't just leave a baby hungry, especially if it got to a point they were concerned about blood sugar levels.