She was responding to criticism from Sir Ivan Rogers, who resigned as Britain’s ambassador to the EU.
In her first TV interview of the year the PM told Sky’s Sophy Ridge she will be setting out her plans for Brexit over the coming weeks.
And she said 2017 is the year change will happen following the UK vote to leave the EU.
Mrs May said the UK needs to question what the right relationship with the EU is.
“I’m ambitious for what we can get for the UK in terms of our relationship with the European Union because I also think that’s going to be good for the European Union,” she said.
“Our thinking on this isn’t muddled at all.
“Yes, we have been taking time, I said we wouldn’t trigger Article 50 immediately, some said we should.”
Mrs May said no plans had been made in the event of a vote to leave the EU so it was important for the government to first look at the complexity of the issues.
And she confirmed that Article 50, the beginning of the formal process to leave the EU, will be triggered by the end of March this year.
:: As it happened: PM speaks to Sophy Ridge on Sunday
The PM told Sophy Ridge she does not see the decision between trade and immigration during Brexit talks “as a binary issue”.
“We will outside the European Union be able to have control of immigration and be able to set our rules for people coming to the UK from member states of the European Union, but we also as part of that Brexit deal will be working to get the best possible deal in the trading relationship with the European Union,” she said.
“Anybody who looks at this question of free movement and trade as a sort of zero sum game is approaching it in the wrong way.”
And she said she remains confident the UK will get a good deal on both issues.
:: Theresa May says Donald Trump’s lewd remarks about women ‘unacceptable’
Keir Starmer, Labour’s spokesman on Brexit, said: “It was very telling I think that she had one question put to her three times and still didn’t answer it, which is: ‘Are you prioritising immigration over access to the single market’.
“I think now 10-11 weeks from the triggering of Article 50 and the most important negotiations for a generation we need more clarity than that and we haven’t got it.”
In a wide-ranging interview Mrs May also said she did not accept an accusation from the Red Cross that the NHS is facing a humanitarian crisis.
And she said US president-elect Donald Trump’s previous comments about women are “unacceptable”.