If you have wooden or tile floors, you may want to get some lino offcuts to stop pup sliding too much.
I with we'd bought a sling to use until pup is vaccinated - by the time I'd realised how heavy he was getting, it wasn't worth it.
I def agree with pp about not spending a lot on collars/harnesses, but I do think it's worth getting a young pup used to a harness and just accept that it will get outgrown very quickly - might be worth asking on Nextdoor.co.uk or FB if anyone has any old ones.
Extra ice cube trays to make chicken stock/puree ice cubes - great to put in kongs.
Can you leave a blanket with breeder so that pup has something to bring home with smells of mum/siblings? When you collect pup, beware they may be sick in the car and take towels/cloths/water/bin bags.
Stockpile/scrounge old towels from friends (I find worn ones are thinner and therefore dry quicker). Useful for everything from drying dog/extra blanket/emergency wee mop up (ours tends to run through it and spread it around the kitchen).
If you're going to do puppy classes and haven't yet booked, start researching (our local ones get booked quite quickly).
Books: Pippa Mattinson Happy Puppy Handbooks and Total Recall (it's never too early to start), plus Easy Peasy Puppy Squeezy
And you haven't posted a photo ... surely it's Mumsnet law 