The existence of that unitary trading bloc was the reason the UK (and others) chose to join in the first place. It's obvious that the ability to negotiate deals as a bloc would be fatally undermined if individual member states were free to strike side deals on their own. Every new member, first of the EC, subsequently the EU, fully understands this and signs up to it on joining - it's not something retrospectively forced on them by an all-powerful EU. It was something the UK actively wanted, not some kind of bitter pill subsequently enforced.
The EU currently has trade deals with over 50 countries, with many more in preparation. The full list is here:
European Union Free Trade Agreements
These are now de facto the UK's agreements. They will case to be so when the UK leaves.
Thank you for the clarification. But there were always special arrangements allowed in the EU. Britain never accepted the Euro as its currency and chose to keep the Pound. I believe the UK could have made a similar case for other issues like trade deals outside the EU but didn’t want to fight for it. Eventually the EU morphed into a bureaucratic monster that became dominated by the Germans to the detriment of most everyone else. Its a mess that its remaining member countries are going to struggle to manage. Wait until President Trump lowers the boom on the EU like he did to China. It’ll be great to see!