The BBC and in part Ch4 are funded by the license fee and therefore don't need to have a hit. Failure is an option which can in theory give more room for creativity.
It's better to say not that failure is an option, but rather than making a season of programmes as they are being produced and axing the show if it is failing, the UK system allows shows to be produced well ahead of transmission but in small chunks so that if it does do badly it doesn't need to be recomissioned.
[1] http://www.channel4.com/4viewers/faq/name/does-channel-4-get...