I would say though the top things people don't realise/aren't told.
Milk doesn't come in instantly, but instead on day 3-5. They have colostrum until then. Colostrum is highly concentrated with immune boosting properties but not much in the way of nutrition/energy. If you're separated from baby, have a traumatic labour or a c section that can cause a delay in milk coming in. You might be advised to supplement in that case. It's a good idea to read about paced bottle feeding or non bottle methods of supplementing if you want to breastfeed exclusively.
That it's not all or nothing. Mixed feeding is perfectly possible, you may want to read about how to protect milk supply.
Milk supply down regulates at around 3 months in causing boobs to leak less, feel less big and full and babies often want shorter feeds at this age. It doesn't mean your supply has gone, it's normal.
Establishing breastfeeding is nothing like established breastfeeding. It gets easier, it is not that intense forever. The longer it goes on the less intense it gets.
How supply and demand works. Boobs are not like fleshy bottles you carry around on your chest, you don't need to wait for them to refill and you don't have a set amount you will produce and then they are empty. Breastmilk production is more like a trickling leaky tap which collects between feeds. Then this tap can get "switched on" (letdown) in response to your baby feeding, crying etc. You may be able to train your letdown to respond to a pump, although most people only get the excess when they pump. So you never need to worry that your boobs feel too empty for your baby. In fact letting too much excess milk build up tells your body to produce less of it as it's wasting resources.
Front line health care professionals in the UK have very little training in bf and may have little experience supporting bf. There are bf expert qualifications and positions in the UK but as they are mostly charities or private healthcare practitioners they are not mentioned by the NHS. But if you want experienced, specialised breastfeeding support look for an NCT or ABM breastfeeding counsellor, a La Leche League Leader, a breastfeeding peer support group or an IBCLC. Google these terms plus your town, there will usually be at least one local to you, possibly with some free or low cost groups to attend.