There's lots of vouchers for size 1+2 disposable nappies. The price then gets hiked hugely for size 3 and 4! So definitely worth moving to reusables and small ones are very cheap second hand.
About 8-10 a day for very small babies, probably 5-6 by 6 months, 3-4 by a year.
Until they start solid food, the poo isn't icky at all and you just need bits of cloth and water (cotton wool leaves fluff...) People do make their own wipe solution but I started buying wipes at 6 months - a multipack of 4x80 wipes is around £4 and lasts me about a month.
I'm not sure my breastfeeding was cheaper than formula in the first 6 months as I needed lots of Lansinoh, a variety of cushions (My Brest Friend was wonderful), and an electric pump, all bought when I got a bit hysterical, so from overpriced Mothercare, but then I had more difficulties than most! For the next year it certainly saved a fortune - up to a year I used about 3 cartons of formula a week so a couple quid.
Baby food is extortionate, so if you make your own - just mash cooked fruit/veg/squashes/banana/cottage cheese/avocado/baked beans/ etc, and try fromage frais, adult porridge oats, weetabix, bread sticks etc, you'll save loads over the equivalent in the baby aisle - just try to get low salt+sugar versions. Probably a couple quid extra a week from 6mo to a year, then somewhat more over the next 2 years as he got fussy and I bought more handy travel packs of stuff.
Activities - look at your library and SureStart centre and local church halls and GP for details of various baby groups - tend to be free or a couple quid max each week. Courses like swimming, yoga, baby signing etc can cost £100 or more for a course. Some activities like mum+pram exercise you can pay when you attend, so less expensive. Your hospital may do postnatal exercise classes involving yoga, which may need to be booked as soon as you've given birth.
Many people meet other mothers at baby clinics/weighing or local group and then arrange meetups at homes or in cafes, so entertainment is cheap.
I found I actually saved a lot of money while on maternity leave - no Travelcard needed, and after the first couple months of lots of preprepared food, I did lots of cooking from scratch and we didn't go out much. The expense started once I returned to work part-time and almost all of that went on nursery plus my tickets, new clothes, ready meals (as more tired), etc - but again still some savings being able to go out sometimes in the week, social life involving people coming to us, etc.
My tip is to hide from all advertising aimed at parents of small children - it's amazing how much crap they must manage to get someone to buy! Admittedly some of it is useful for some people in some situations, but do think hard before buying stuff...