The issue is what level of tariffs the EU insists on, and..impact..on existing trade.
Because worst case analysis is all UK goods going to the EU are priced out of their markets and the UK retaliates.
Yes I think that would be very childish. Didn't stop Trump putting 25% on steel imports to the US, did it?
This is the sort of scoping exercise David Davies should have done but didn't.
It would have answered the question "How well can the EU really do without the UK to export to?" The answer is "quite well." It would also answer how well the UK could do if it couldn't export anything to the rest of the EU. The answer is "not very well at all."