Well why would the UK impose punishing import tariffs on itself?
Without a trade agreement the importing country is free to decide on how much to charge if anything.
Several countries Hong Kong, Singapore, Dubai have zero tariffs on imported goods.
On the other hand without a trade agreement the country you export to can charge as much as it wants!
Which is a major problem for exporters, but, all the project fear panic is directed at imports which would seem to be a non-event. It all seems like a re-run of the pathetic Y2K non event remember "Millenium cupboards" now we have "brexit cupboards" .