Please could you tell me if it is possible to get married outside, in our garden, for example. How do I get permission and the services of a civil marriage person/vicar?
Unless you live in Scotland, it isn't possible to legally marry in your garden in the UK. If you want a civil wedding you have to marry in a venue licensed for civil weddings, either a register office or a hotel or stately home or licensed place of interest (for instance a zoo or football ground.)
You could have a civil ceremony before your chosen wedding date, and on the day have a humanist wedding in your garden. Humanist weddings are not legally binding (except again in Scotland) but can be suitably personal and spiritual.