well. before you start thinking about salary maybe have a deeper thought into what you want to actually do. because average salaries in economics aren't going to be very helpful to you if you can't convince a job interviewer you want the job
dentistry is a tough degree, not just because its 5y, and as with medicine, if your heart isn't in it it's probably not wise to press ahead with a gap year. i definitely wonder about your motives here, although i appreciate its difficult to find your calling in life whilst still in sixth form
economics is a mainstream employable degree, good for plenty of professions. but then so are most degrees. ucl is a great school, target/semi-target for finance, so that gives you something of a leg up in the jobs world