An MP's constituents come after the responsibility to the nation as a whole do they not?
An MPs responsibility is "in the best interests of the country"
You are confusing, what people vote for with what is the best interests of the country.
Brexit was never in the best interests of the country, only the vested interests of Tory MPs, disaster capitalists, hedge fund managers, right wing libertarians, fossil fuel interests, Russians and any other shyster.