It depends what you seems value for money to spend money on, how your child will develop and what is seen necessary in future years.
DD is 15 and I think the most expensive years where before primary school thanks to nursery and now that she is a teen.
Pocket money - you choice if you link it to chores or anything else. We don't. DD gets a decent allowance to spend on all things she deems necessary, we see extra. It teaches budgeting and really thinking about purchases.
Technology - absolutely necessary in secondary and at least access to it in primary school.
Hobbies - schools don't really teach swimming and playing musical instruments. If that is important to you, then you need to pay privately for it.
For us, sport is a must but it varied between £100/month riding and £75/term recreational gymnastics. If you have a girl and go the dance route, expensive outfits, show outfits, exam fees and extra lessons for said exams are basically the norm for most dance schools.
Clothing - DD outgrew most children sizes when she was 11, adult sizes are not VAT exempt. Same with shoes. Luckily DD is not into brands or anything, she has trainers from Deichmann and shops at NewLook/H&M. It will change when she is at 6th form.
Uniform - primary is cheap, secondary is an expensive one in the begining but then gets cheaper or similar to primary as they don't grow that much anymore and also don't destroy their clothes on a regular basis.
Car seat until they are old enough/tall enough. Maybe a bigger car if you have 2-3 in car seats at the same time.
Braces - I agree, NHS is very difficult to get onto the lists.
Room - you will need to change room furniture from baby to child to teen. Some things can grow with them but be prepared to spend again.
Tutoring, not just for entrance exams or 11+ but just to catch up in some cases.
Days out/holidays - most theatres don't do children discount, we love seeing a show but it means 3x the ticket price. Lots of places aimed for children make parents pay also a small fortune. Holiday can be expensive if you don't want to spend the whole time in the same room as your child, a suite/two bedroom apartment are suddenly costly.
Driving - that is lucky covered by money from the grandparents instead of lots of expensive birthday/Christmas presents.