For those asking about having formula when planning to breastfeed I would second the good advice from @Gigitree
Giving a top up or a formula feed is not a failure or caving in, you don't have to put yourself through hell to successfully feed your baby. I did that with my first and it didn't end well (hello PPA). Successful feeding is when everybody, including Mum, is happy and have full bellies #bottlesboobsortubes 
As @Gigitree said, supplementing with formula can be a really helpful if you (or baby) are struggling with BF and need a break. Research shows topping up in the early days can actually help prolong breastfeeding.
Having done the midnight dash to the 24hr Tesco with no idea of what to get I would personally recommend having supplies in, or at least having a think about what formula you would like to use if you decide to do so and making a list prior to when the sleep deprivation and feeding struggles kick in.