The first episode in an eight-part series attracted a 19.2% share of the audience, making 'Headcases' the second most-watched programme in the 10pm slot, after 'BBC News'.
The BBC's flagship news programme drew 4.6m viewers and a 21.2% share.
'Headcases' was backed with a marketing campaign that included on-air trails and previews in a wide range of newspapers and magazines.
It received mixed reviews from TV critics, with many agreeing that 'Headcases' lacked the satirical bite of 'Spitting Image', which it is aiming to emulate.
Andy McSmith at The Independent, said: "The quality was dazzling; the only thing in short supply was good jokes. So Posh Spice has ambitions beyond the range of her talents. It is not what you call breaking new ground in satirical humour."
Earlier in the evening, ITV1's last episode of three-part crime drama 'He Kills Coppers' was slightly up on last week with 2.6m viewers and a 10.5% share at 9pm.
On BBC One the second episode of a three-part 'Casualty'-special, set in 1907, brought 5.3m viewers and 21.6% share at 9pm to win the slot, although it was down by more than a million on last week's 6.7m.
The final part of Channel 4's 'Stephen Hawking: Master of the Universe' pulled in 1m viewers and a 4.4% share at 7pm, compared to 860,000 viewers last week. Including Channel 4+1, the show netted 1.1m viewers and a 4.8% share.
Channel 4 also earned a respectable 2.7m viewers and a 10.7% share for Hollywood action movie 'Sahara' between 8pm and 10.20pm. Including Channel 4+1, the film was watched by 2.9m viewers and a 11.5% share.
Earlier in the day, ITV1's 'F1: Bahrain Grand Prix' live coverage won 3.1m viewers and 29.2% share from 11.30am-2.30pm.