The two other answers to this question are spot on, but I'm going to interpret this question in a different way. I'm going to answer it as if the question said "Who was the first presidential style Prime Minister of UK?"
I would argue that there have been two 'Presidents of the United Kingdom': Margaret Thatcher and Tony Blair.
For the first eight years of her administration, Margaret Thatcher was effectively 'the President of the United Kingdom'. Her administration was able to do things most post war PMs were not able to do, possibly buoyed by the large mandates she was given by the British public in 1979 and 1983.
Given the landslide election of 1997, it became almost impossible for the Conservative party to win the 2001 election, and very unlikely that would would have much of a chance in 2005 (Michael Portillo's words, not just mine). With this sort of a political landscape and public mandate, Blair was able to govern as a de-facto president, allowing him to push through parliament decisions that didn't have, not only, the public's backing but even the backing of much of the Labour party. This can be seen in Blair's decisions regarding Iraq and Afghanistan post 9/11.
