I do understand where you are coming from, at 39 I was single, in a job I hated, still hopeful for children in my future.
I don't thinks its unreasonsable or selfish of you to want to change your life and be a mum, but maybe there are more ways to skin the cat.
If you currently command a salary of £45 k, I assume you have qualifications and transferable skills which could take you into a more child friendly job which would give you an income
When you are thinking about having a child, I'm afraid its not all about you. Of course love and time are important, but its not enough. As I said I don't have an extravagant life, its more than 'just enough' but I can't afford many things I would like to give my son, but he will never be hungry or cold, and will always have enough 'stuff' so that he does not feel excluded from the life his friends have, in fact he does a lot better as an only child than his best mate does as one of five brothers in a two parent family.
Of course you can make it work as a single parent, many of us do, but you don't need to take a nose dive off a financial cliff. You have assests, qualifications, why not use them a bit more effectively to support your goal, rather than literally go for broke.
And everyone needs support, of some sort, be it practical or emotional. I have a lot more support now because I have made new friends in my life as a mum, I've aquired a lot of friends in the last 6 years. You do need to be open to the fact that you will need support and to ways of finding that support