The opening two sentences from this news item says it all:
"The growth of digital radio’s listening share in the UK has continued to be minimal, sparking concern that the government’s switchover decision date is unrealistic. The new radio audience figures for the last three months of 2011, released today, show that digital listening now accounts for 29.1% of all radio listening. Digital radio listening must reach 50% of all listening by 2013, if the government can proceed with the switchover process."
http://www.rbr.com/radio/uk-digital-radio-numbers-not-pretty.html
This also explains why an analog cut-off date for radio in the US would probably not work.
"The growth of digital radio’s listening share in the UK has continued to be minimal, sparking concern that the government’s switchover decision date is unrealistic. The new radio audience figures for the last three months of 2011, released today, show that digital listening now accounts for 29.1% of all radio listening. Digital radio listening must reach 50% of all listening by 2013, if the government can proceed with the switchover process."
http://www.rbr.com/radio/uk-digital-radio-numbers-not-pretty.html
This also explains why an analog cut-off date for radio in the US would probably not work.