At the moment the government childcare voucher scheme only covers employed parents and is also only valid if employers take part. However, from late 2015 the scheme is being changed. In the future any household with a total earned income of under £300,000 will be eligible, and this also applies to self-employed people. At first this scheme will only benefit people with children under 5, but over the next few years it is supposed to be extended so that it covers children up to 12. THe vouchers will not apply to households where one parent is staying at home - you must both be working or a single parent.
Once the scheme is in place you should be able to buy childcare vouchers online and the government will top them up. They can be used with any Ofsted recognised childcare provider. You can learn more about the scheme here.