The problem with this piece is that it isn't true. In the first paragraph Tim Ross refers to 'withdrawing from the European Union's single market'. In the third paragraph he refers to Britain being unable to make its own trade agreements 'because it is part of the EU's "customs union"'. He is confusing readers by conflating two completely separate entities. Confirmation that Ross is all at sea is when he goes on to say this:
Dr Fox is expected to signal his strong intent that once outside the EU, Britain will take up a fully independent place at the WTO and will be free to strike competitive deals with countries across the world.But the customs union is the European Union itself. So once outside the EU we would no longer be part of the customs union. Further, the EU doesn't belong to the single market, which his comment suggests. A country can be outside the customs union but participate in the single market. Three countries already have that position, Norway, Iceland and Liechtenstein. It would be nice if Telegraph journalists tried to understand these things before publishing such confused nonsense.
In order to achieve this, Britain would have to leave the EU single market’s customs union.
So all Liam Fox will be doing is reiterating that Britain will be leaving the EU, will therefore no longer be part of the customs union and will once again, by default, speak with its own voice at the WTO.
Fox will not be saying anything about continued participation in the single market, because no formal negotiation or decision about Britain's post-EU position has even taken place. If he were to say something so premature, you can be sure Theresa May will be dancing a Tarantella on his head shortly afterwards.