Edward Thomas was the production designer for almost every episode of Doctor Who from series 1 to series 5. He was also the production designer for every episode of Torchwood since series 1 and for series 1 of The Sarah Jane Adventures.
Therefore, his influence on the look of the Doctor Who universe cannot be overestimated. It goes only a small way towards explaining his importance to note that he designed the TARDIS interiors and exteriors for Christopher Eccleston, David Tennant and Matt Smith, the Torchwood hub, and Sarah Jane Smith's attic.
After The Big Bang, he relinquished his duties to Michael Pickwoad, then became the production designer for another BBC science fiction programme, Outcasts.