The World Trade Organisation does not set tariffs, it's individual members do. At the moment our membership of the WTO is managed through the EU and all of our international trade arrangements are EU trade arrangements. When Brexit is implemented we will set our own tariffs but it is a fundamental condition of WTO membership that you cannot impose different tariffs on different countries without a free trade agreement in place. This is beneficial to the UK because it means that EU countries won't be able to impose punitive export duties on products they sell to the UK without also putting those duties on every other non-EU, non-EU free trade agreement country (such as the US, Japan, China, Australia etc etc). The quid pro quo is that we can't pick and choose which countries' products we out tariffs on, it's all or nothing.