fwiw - I'll add my 2p.
Hampstead - flying into Channel Island CAT is a breeze (as long as they have your flight plan). They are v helpful & friendly, will clear you in and give directions to join. Just follow instructions and u r sorted. If your not used to it, it a great feeling going to a big airport with large passenger jets around you.
wrt the "special branch form". I haven't used one in years. I just send a fax with the flight details (dep/ arr time, place, destination , a/c & pilots name& contact number) plus Names, Addresses, DoB, Place of B, Nationality & Passport Numbers of all on board. They don't seem to care how its laid out as long as they have the data