Hello, This sentence needs a few changes: "We voted to leave the European Union as we believe it is important to develop an independent economy in the UK." -You could use "Brexit" as a verb, but it would be informal and a bit uncommon, so I replaced it with "leave." -"to draw a line under" is a bit of an odd expression, so I replaced it with "develop." If you wanted to keep the original meaning of "draw a line under," you could use the verb "underline" or "emphasize," but I think that "develop" is most appropriate in this sentence.

--> We voted to leave the European Union as we believe it is important for the UK to have economic independence. Note: You can't vote "to" something. We would say 'We voted for Brexit".

"We voted to quit the Brexit from the European Union as we believe it is important to draw a line under an independent economy in the UK".