Hi Dizzy,
Actually I'm at Cambridge Uni right now and I would hugely recommend it. The level of tuition is some of the highest in the world and it is consistently ranked in the very top in international league tables. Of course cost is a big issue, but my college (st. John's) certianly offers certain help and grants. For example you can get a huge amount of help with your costs for things like study materials and books, and every student automatically gets half of the money they spend on books reimbursed by the college every term. I also became a scholar at the end of my first year and yes you do get financial assistance as a result of winning a scholarship. My friend is an international student from Athens and the college has been hugely supportive of her financially. You have an individual tutor who you can go to to talk about financial issues and the college is often able to help. There are also several university wide prizes, grants and scholarships available to help with tuition fees, I have friends who have benefitted from them, and within the college there are prizes awarded every year from bequests from past students and alumni, often specifically targetted at helping foreign students or those who are struggling financially, and you can apply for all of these.
In terms of your application, Cambridge obviously looks for extremely high academic results, so absolutely push yourself to achieve the highest possible grades, but remember that your extra-curricular activities are also taken into account, so if you can prove you have been involved in activities at school that involve teamwork or responsibility or leadership etc. those are always good to do so you can mention them in your application form. Also whatever subject you decide to study it is really helpful if you can show that you have made the effort to do some work experience or shadowing in that area, as that is likely to aid your application also. Most of all don't give up or be discouraged, it is one of the top universities in the world as the rankings consistently prove, and has 80 Nobel prizes to its credit, more than any other university in the world, so why not reach for your dream?
Really really good luck, I hope this helps
Laura