The UK Passport Agency are the people to deal with. Their website is very informative, and you can even prepare a passport application on-line (you still have to print it off, sign it and get the photographs, of course)
http://www.ukpa.gov.uk/
Sorry - I forgot just what happens with the on-line application. THEY print it off and send it to you to sign and return with the photographs.
HOWEVER
I think it might only apply to renewals.
I did an on-line renewal when the facility was still being tested.
The new passport arrived by return of post.
If you are in the UK you can also ask for an application form at a main Post Office; some also offer a service whereby they check your application is accurate and complete before you send it off (there is a fee for this). I used this service when I was applying for my daughter's passport and it was money well spent because they spotted something I had done wrong which would have delayed my application.