Well firstly you should not be worrying about your basic understanding of economics because UCL does not specify a high school study of economics as a specific subject. Therefore, with your 'basic' knowledge, you will be ahead of some people. Also, the first semester at the university is all about catching people up to the same level so you need not worry about the differences between your understanding and others.
UCL is a prestigious university and the focus on a study of economics will get you far. The advantage of studying a subject in depth rather than studying a broad range of subjects is that employment is much easier after and the need for a second degree is much smaller. So as long as your sure economics is the subject for you and you are happy to study it exclusively for the next three years, you are more than likely to get a really good career.
London is one of the most culturally diverse and interesting cities in the world and you are sure to have a good experience. In terms of employment, being in the capital, there are many large fims recruiting every year and they target undergraduates. Also, an economics degree is probably the most highly acclaimed by companies and especially investment bankers
Hope this has helped and I hope to see you at UCL in September