I think the reason why "Bremoaners" make the opposing point about Brexit is typified by the lack of comprehension of basic economics and politics in your post. Firstly, it is important to remember that the UK hasn't actually left the EU; the current turmoil in the economy (pound being at 163 year low, investment halted, inflation rising, GDP growth slowing etc) is based on what the markets expect IF the UK leaves the single market. As the UK is still in the single market, the full impact of the decision to leave the EU has not been felt. Essentially, if you think the economy is doing well post-Brexit: you are wrong.
This nicely leads to my second point, you need to read a book if you think a fall in currency is not a bad thing. A devaluation in the Pound means Ā£1 is worth less compared to other foreign currencies. Therefore, everything we import (which is a lot) becomes a lot more expensive, hence inflation. This will affect everything from the weekly shop to trips abroad. Essentially, our national wealth just fell by 20%. This may, for a short period, boost our exports, but once exporters lose privileged access to the single market, their goods will become relatively more expensive - cancelling out any perceived benefit. Essentially, please do not read the Express or Daily Mirror for economic lessons; read a book or a professional journal.
Thirdly, the fact that Marmite is made in this country is irrelevant. Many of its constituent ingredients, from the plastic jar to the gas which transport it to the supermarket, are imported, which now due to the fall in the pound is more expensive (see above).
Lastly, the fact that people point this out, and continue to make the opposite case for Brexit is part of our democracy. One of the supposed reasons many Brexiteers voted the way they did. So labeling people Bremoaners or accusing them of "talking the country down", because they highlight the inconsistencies or outright lies from the Brexit camp, just highlights the fact that you do not have anything intelligible to say.