" We’re sorry to let you know that the Post Office mobile service will no longer be available after 8th August 2016.
We’re aware that all our customers will need some time to use up credit and move to another provider, so your last date to top up your phone will be 22nd June.  If you want to move to another provider and keep your number then please contact us by the 4th August to request a PAC.
Please be assured that any unused credit on your phone will be eligible for a refund and further details on the process will be published here at a later date."

https://mobile.postoffice.co.uk/we-are-closing/

Why is the service closing.

"Since June 2015 Post Office has offered a "Pay As You Go" mobile service online and in 263 of its 11,600 branches, but has now decided to conclude the trial as the results did not give us sufficient confidence that mobile will contribute to our goal of commercial sustainability. The money needed to invest further in the mobile could not be justified at a time that Post Office is investing in modernising our branches. "