Mother I expect that a no-deal Brexit would bring a certain amount of short term chaos everywhere
- but especially in the UK -
since it would be basically taking the #5 economy in the world offline, out of the standard international trading, certification & trasnport system.
Short term, it would probably mean the Uk not applying any tariffs, unless / until the govt has sorted out its WTO quotas etc
Other countries would probably choose to apply standard WTO tariffs for 3rd countries, against UK exports
So, that would hammer the UK trade deficit and disadvantage some UK producers, partly offset by a probably much lower pound
-
imo, financial markets are still assuming there will be a WA and transition period,
but if the EU / UK states no deal is going to happen, Sterling would plummet.
However, the WTO itself doesn't really impose sanctions very speedily on countries which transgress and iirc requires proof of harm, rather than just breaking the tariff or MFN rules
Also, the WTO disputes court has been seriously hindered by the USA refusing to approve judges for it - this went back even to Obama's time iirc - the USA in practice always hasthe policy of "we have only interests, not friends"
So, in practice, the UK could proceed as it considers in its national interest re imports & tariffs;
but
if countries felt the UK actions were disadvantaging them, they could then apply punitive sancions on selected goods,
e.g. as China & the EU did vs USA goods, about a month after Trump imposed his unilateral tariffs
- and the USA is already suffering; the UK would probabl;y be much more vulnerable to punitive actions.