Wikipedia says of the design:
Boyle designed the gold-coloured space suits along these lines despite persistent encouragement to model them after the NASA template. The helmets were designed to have cameras mounted in them. This further enhanced a sense of claustrophobia useful to the actors in delivering more heartfelt performances. The helmets were also limited to a horizontal slit for visibility instead of a full-face visor as further consideration toward protecting the characters from the ambient radiation of outer space. According to Boyle, the funnel shape of the helmet was influenced by the character Kenny from South Park.