Am I not going to argue that the WTO rules aren't benign. However, just saying "we now trading under WTO rules" doesn't mean anything yet. You actually have to do some work.
In essence, you have to set schedules: you have to set
That is, you have to define what percentage must be paid on imports (tariffs); you have to define how much you are allowing into the country (quotas); and you have to define how much you want to spend on subsidies. And you have to define this for each type of product.
Before you think "how hard can that be?", you have to realize two things. First, you cannot pay favourites. These schedules apply to any country you don't have a trade deal with (trade deals usually take years to negotiate). You cannot apply a different tariff for the USA than you can have for Russia (not until you have a trade deal with one of them). Second, every country who is a WTO member (so, essentially, every country) can object to the schedules. Which will lead to disputes which can take years to settle.
One may think "the UK can just copy the EU schedules", there are problems with that. First, its the quotas. The UK certainly doesn't want to allow as many products into the UK each year as are now allowed into the EU. Second, it won't have the capital to hand out the same number of subsidies as the EU does now. The UK is just so much smaller than the EU.
And the UK being smaller than the EU is the real stinger. The EU as a whole has huge bargaining power. It has high tariffs on agricultural goods. It gets away it. Would the UK set the same tariffs, countries like the USA, Brazil and Russia will object. It's in their interest to have much lower tariffs.
The UK certainly doesn't want to trade under WTO rules with their biggest trading partners. They want to strike trade deals. Brexiteers think they can make better trade deals than the EU currently has.
And we've all seen in the past two years how good the UK is in negotiating deals.
Here is a video explaining WTO rules.