The reason Lady Penelope's car was not a Rolls Royce, as in the original series, is because Rolls Royce were concerned that should the film be a flop then that would affect its sales and reputation. The film did flop in the U.S. box office.
Lady Penelope's pink car is not a Ford Thunderbird at all. The car was designed and made at Pinewood and Shepperton studios, using a specially designed engine, running gear, etc. Ford let them use the design for the front of the car from their current Thunderbird.