Like another poster above I also don't understand why it was necessary or relevant to bring your age into this at all.
You posted this bit:
I'm 18, just recently turned 18. I'm trying for a baby with my boyfriend, we have been planning this all for the last 6 months, this really is what we want, we have a good income (we don't currently have any benefits and we plan to not rely on them)
When all you really needed to say was this bit:
I am trying for a baby. I came off my mini pill 1 week ago, I'm having heavy bleeding now, I presume this is my period.what do I need to know, what's important. I've googled and googled but I want really advice. How long do you think it will take for me to fall pregnant etc?
That would have given you all the practical advice you needed with no judgement.
You say you didn't come here for life advice, that you are not interested in listening to people's opinions or having to defend your choices. Fair enough.....so why completely unnecessarily provoke people into a making those judgements by furnishing them with all this apparently irrelevant background information? 
I don't think you sound especially immature, no more or less than any 18 year old, but the wording of your first post gives you away rather. It is not really the work of a well rounded adult with their head screwed on, I'm afraid. It sounds like the work of a naive, stubborn, idealistic teenager trying to convince everyone they are a mature adult by saying 'look how sensible I am.'
The irony is that it shows us who you really are, (the teenager) and not the person you are trying to portray yourself as (the mature, got-it-all-together adult.)
I am also a bit confused about these jobs and qualifications you have, and the timeframe that all this has happened in.
You are only just 18, you say you started doing AS/A levels and that your exams 'were not good.' So did you switch to the diploma after doing too poorly at AS to continue to A2, or did you switch without finishing AS levels? Aren't diploma courses usually 2 years? If you had some offers from unis that can only have been for this coming academic year due to your age, so surely you are only just finishing sixth form college now like most other 18 years olds? 
So how have you managed to do that and fit in a full time job in a bank? You can't have worked there for very long at all, it just doesn't make sense. 
Also, I don't know how old your boyfriend is or what he does for a living but that is some seriously good money you are apparently earning between you there. Frankly, it's a bit unbelievable. How long have these amounts actually been coming in for, and are they stable jobs with contracted hours?
Anyway, look....whatever the answers to any of the above, it makes no difference; fact, fiction, fantasy, wishful thinking, massaging or the truth, whatever.
If you've made up your mind you want a baby you will go ahead and have one, no matter what anyone older and wiser tells you, and even if you are actually in the most ridiculously unstable, emotionally dysfunctional, poverty stricken and inappropriate set-up ever because (sadly) that's what many teenagers (and plenty of stupid adults) do all the bloody time. Their primal, biological, selfish urges somehow trample over any tiny bit of common sense they may have and they JUST CAN'T WAIT so they do it anyway. Because apparently, so long as you luv ur bubba wiv all ur heart for ever and ever, everything else will just magically fall into place. 
So good luck with that. But be prepared.
Be prepared to probably not be with your baby's father by the time you are 22.
Be prepared to hate his guts in a few years as you struggle as a single mum in a grotty rented house and he leads the life of Riley, making up for lost time and those years of feeling 'trapped' and 'tied down too young.'
And then be prepared to sit back and watch as he settles down and has kids with someone else, and suddenly he can't give your child much money or much attention anymore, because he needs it for his 'wife and family'.
Be prepared to never take that uni place, or to feel completely mentally and emotionally exhausted as you juggle taking your degree with being a single mum.
Be prepared to always be a bit broke and a bit fed up and a bit resentful of other young women your age with their glamorous, carefree, fun filled lives.
Be prepared to go through long periods of loneliness, punctuated by a succession of useless boyfriends because none of them will commit to you and your child, while you watch other women your age travelling the world, having great social lives and then settling down and buying lovely houses with great blokes with good jobs, and having the fairy tale.
And most of all be prepared to feel a bit foolish when you look back and remember this thread. Because all the stuff I've just said there ^ is the harsh reality, for many many women who thought they knew it all at 18, and didn't see the point of waiting for a baby, just like you.
Hindsight truly is a wonderful thing, and the most wonderful thing of all is that you have the opportunity to learn from other people's. But you won't. No-one ever does.