Hi Paul, how's the new toy?
As far as I can make out, for a private/pleasure vessel, part 1 registration involves a lot of forms and money and allows you fly the Red Ensign.
The SSR involves one form*, less than thirty quid and allows you to fly the Red Ensign...
You pays your money and makes your choice
(Unless you've got a marine mortgage the size of Brazil's national debt, in which case the financiers may help you in your decision making
IMO = International Maritime Organisation. You can have a bigger registration number. Just like a real ferry...
*2 forms if you include your CG66, which you've done already. Haven't you?