You need to start to put money away now to supplement your SMP, that is what most people do. You also need to look at finances from a joint point of view as you will be a family unit.
You need to look at areas where you can cut back, takeaway lunches, teas, coffees etc, nails, haircuts, phone, gym, hobbies, tv packages (ignore anything not applicable).
I stopped getting my hair done every 8 weeks once the baby came along. I bought clothes for the baby rather than myself. Your priorities will change and so will your spending habits.
when I was married, XH and I paid everything 50/50 until we had a child and then it became impossible to do so as I wasn't earning enough money. We saw ourselves as a family with pooled finances and it wasn't a question of who paid for what. He didn't see himself as supporting me as I was at home looking after his child.
You get SMP for 39 weeks then unpaid leave for a further 13 weeks. You will accrue holiday pay during this time so you could ask your firm to pay that to you over 2-3 months to enable you to take more time off.
I don't see people being harsh to you, I see people giving you a reality check. You can't expect the government to pay you a full time wage to have a baby as a child is a lifestyle choice and it is up to the parents to ensure that they can afford to have one. They say that in reality, nobody can afford to have a baby, yet millions of people do every year. A friend of mine went back to work when the baby was 6 weeks old because they had a huge mortgage to pay and she was a high earner.
Most employers do just pay SMP and it is taxed as any other income is. If your income drops and you have overpaid tax, then as others have said you may get a tax refund each month.
Look around for good condition second hand baby equipment (facebook, ebay etc) buy basic baby clothing from supermarkets like Asda which are cheap. Charity shops are great for toys and books as they grow up. It doesn't have to cost a fortune to have a baby.