A comfy nursing bra (doesn't have to be expensive, Asda do a pack of 2 for £10 which are very comfy!)
A flask or water bottle is useful, especially when cluster feeding! Something like a Chilly bottle which keeps drinks hot or cold is fab, and can obviously use it when out and about too.
A dressing gown if you don't have one already, not directly BF related I guess, but during night feeds it's so cosy.
Breast pads and nipple cream, I would just get a few to start with as you don't know if you'll need to use them for long.
Pick some box sets and films you have been wanting to watch and make a list, I was so tired at the time I never knew what to watch.
Prep some meals or get some easy to cook freezer food beforehand, it will make life so much easier in the early days! Plus some oatcakes, oats (if you like porridge) as theyre handy snacks and also supposedly help with supply.
Muslin cloths for cleaning up milk/spit up are so handy.
Anything else you can buy as and when you need it, if you find you do need/want a pump a lot of hospitals rent them out for cheap (here you just pay £5 for the sterile parts); and if you decide you want to use a Hakka or some bottles etc can order for next day delivery. A comfy cushion is also useful, although I found one I already had was fine.