From the website:
How the Challenge works
The Spending Challenge is an open invitation for public sector workers to contribute to the government’s Spending Review, helping to rethink public services to deliver more for less. It’s open initially to public sector workers between now and July 8th, when it will be opened to the wider public.
The Prime Minister and Deputy Prime Minister have committed that the government will look at every single idea that comes in. All the suggestions will be considered by a team of officials at the heart of government. The final Spending Review, which will set out detailed spending plans for all government departments, will be published on October 20th.