This is how international trade works. It's intimately intertwined with diplomacy and each party will play according to its own interests, related or not.
Just like Russia could say "no gas for you" if you make too much of a fuss about what happened in Crimea.
Now if the UK's view of its own future diverges from the EU, it's totally normal to see complications appear during negotiations.
After leaving, the UK will be in a weak negotiating position. Recession is almost certain, and even using an existing trade deal like CETA as a basis will take multiple years to negotiate because of the current UK red lines, like controlling their own immigration policy.
Canada had to adjust their immigration policy for several EU countries to get CETA. The UK will be over a barrel and will have to make major concessions.
Whether you're pro EU or not this situation is a total balls-up.
Watching the UK negotiating a trade deal with the USA and China at the same time, considering the current geopolitical climate, will be outright interesting. Also take the HK situation into consideration.