Really I think as many maternity pads and nappies as needed should be provided by the NHS, free at point of use, during your stay in hospital. After all we don't take in our own bandages and antiseptic when we have an operation. Let's begin from what the current situation is however and accept that they aren't.
But even if they aren't free, they should be available. No one knows how long they will have to stay in hospital post birth so no one knows the quantities they will need. And no one wants to have to send their partner, parent or friend on a hunt to find supplies when they should be with you.
Have a shop in each maternity unit, or even a trolly going round the beds, and sell maternity pads, nappies, wipes, cotton wool, nipple cream and even shampoo and toothbrushes. And do so at high street prices, not vastly inflated prices.
If you can have burger bars in hospitals then how about a branch of Boots or Superdrug - it really would make life easier.
Ways to improve postnatal care #1: Don't call me mum
Ways to improve postnatal care #2: Change the sheets
Ways to improve postnatal care #4: Allow partners 24 hour access
Why I love the NHS