Be careful about the eyewitness report.
Having done that takeoff several times I can tell you that the trees immediately behind the garden centre - which the plane cleared - are on a slight rise and are the highest point on that heading. If the plane was continuing to climb or even just maintaining altitude it should have cleared the next row of trees.
But there are hills only a mile or so ahead and the pilot would either have to gain another 600 feet on that heading or 100 feet after a turn east to clear Shanklin and get out over the sea. So he would have been desperate to gain height.