Is the UK Global tariff now set in stone or can it change if there is a deal with the EU?

I am seeking clarification on whether the UK Global Tariff will apply whether we get a deal with the EU or not. For instance if a chemical material is going to be 0% to import from USA to UK based on UK global tariff but the EU still have to pay 5.5% for same material, surely if there is a deal done they would not agree to UK having preferential duties which could effect the EU Market? Basically I’m after something official that confirms the UK Global Tariff will apply regardless of what happens between UK and EU in negotiations in next 6 weeks! I understand duty rates between UK and EU may change if a deal is done, but it is more to do with importing from outside EU such as USA and China that I’m referring to.
Many thanks