The extra a level will not be overlooked. But, finance isn’t that competitive; economics is. You can get into any uni for finance without further maths (with a relatively high probability).
But, bear in mind that if they say they need for example A*AA and you don’t get that in your 3 that you studied in the 2 year period (so for example you get A*AB and you are predicted A/A* in the a level that you are studying in your gap year) they might not accept you.