That sounds okay to me. The few people I know tended to travel for a year and then secure a gap year scheme for the next year. You just don't want to be working in audit for the whole 2 years or something like that. As if you plan to eventually go to university, that second year doing the same thing will not help at all in applications (the first year is enough for some finance experience) and if anything, just makes you look a bit confused if you happily spent 2 years in audit/tax etc then applying to front office.
The only other thing to think about is that you will be 2 years older once you get to university. In terms of securing a graduate job from a summer internship, you tend to only get one chance (between 2nd and 3rd year). So if you were unlucky and got some desks with no headcount, and hence got no offer, you would be forced to do another gap year or a masters the year after (unless you're confident in applying for graduate roles where each bank takes roughly 2-5 people per year across front office at the moment). So that would mean that you would be 6 years out of A levels potentially before you get a job. Just something to think about - it may not bother you hugely, but I know people who are a bit older at uni who are itching to get working. On the other hand, Europe might be growing 3% a year once you're looking for jobs!
Either way, you won't be disadvantaged by taking a gap year to get some experience.