assume less context in service.gov.uk intro

The introduction to the service.gov.uk page assumes context on the part of the reader about what kind of services we're talking about, and why they might live on service.gov.uk subdomains. This is an attempt to provide that context, but as I'm not sure I understand it myself, I'd appreciate feedback on this -- particularly from @jystewart, @garethr and @ejhp.