Hey i'm in the same kind of situation, completely underachieved at school (though still got a few good grades) and want to take the leap of faith and go for it at uni. I've spent a long time researching where to go and after going to its open day on saturday Greenwich really, really excites me.
The grounds are stunning and the local area cant be beaten. South London intimidates a lot of people, especially these soppy sods on here who get nose bleeds whenever they hear someone say 'init', but if you actually realise whats on offer its an extremely rewarding, diverse and exciting place.
The uni has a very modern learning environment (ok, in listed 17th century buildings) which puts a lot of traditionists backs up, it suits me better as i like a lecturer to be more down to earth and aproachable then feeling like im in the presence of God. It has every resource that i really want or need and, as you say, you get what you put into it, and if anyone tells you that you'll get a poor degree at greenwich even if you work hard, they are deluded.
You'll often hear about league tables and drop out rates when talking of negatives about greenwich on a lot of forums. If you base your decision on just a newspaper league table, you're in trouble. They are meant as a guideline - not specialised to your indiviual needs, when i calculated what i wanted from a university Greenwich actually beat everywhere within my -260 tariff. The drop out rate, you have to look at students who attend the uni, it has a large state school, lower priveleged body that hasnt had as many opportunites academically and the uni prides itself in helping these students, but obviously the uni can only do so much. How others do shouldnt really effect you though, your degree is what YOU make of it.
The halls, facilities and lecture rooms are modern and comfortable, the finacial help you recieve is great (the students affairs liason told me a lot of beneficial things i hadn't heard anywhere) and the pricing of accommodation isnt that bad considering its location.
There are negatives, as any uni in our tariff, but the place has SO much going for it. Go with your gut feeling, if the course sounds right for you just go for it.