-You start applying around the day you want to move there.
-If you go in person you will get it the same day,if you mail it ,it will take a few weeks
-Yup heres the link to make your appointment
https://www.britainusa.com/eappointments/losangeles/ -If he has a creditcard he can make the payment on the site. They don't accept UK checks and I don't think they accept any checks either for that matter. If he doesn't have a credit card you can make the payment at your appointment with a money order or your credit card
-You can use a company if you want but it would be a waste of money, As an American applying for it,it will be much easier for you. I think most if not all us here, did it ourselves without a company
-I'm sorry I'm not getting what your asking? are you asking what reasons they have to deny you? or what reasons they have to stop you at the airport customs?
-yup he can
-Get all your paperwork ready before you do your application. Heres what you need
http://www.britainusa.com/visas/other_show.asp?SarticleType=25&Other_ID=320