Under WTO rules you can't favour one country over another by charging a lower tariff on particular goods, just in order to "do a deal" with them.

That is the rule and we successfully do most of our trading by those rules. No-one has suggested anything different.
It means that a vindictive EU will not be able to discriminate against us as you suggested they might.

And not everyone is going to be enthusiastic about doing deals with us, or even going to be caring a jot.

What is your reason for stating that? It is made up! We are one of the biggest economies and traders in the world. Why would anyone not want to trade. What countries choose not to trade with UK? None.

In terms of goods we are always going to buy more than we sell

Yes, and that makes us an ideal and highly attractive trading partner.