The address lookup isn’t a great experience, and certainly not accurate.
Can’t type a postcode into the Find Address box (which is what I expect many will try to start with).
Typing my full address in (I’m in a flat) doesn’t return my address to select from, it comes up with something similar in the same town but 2km away.
Clicking through to the If you can’t find your address, click here form just about generates the right address, but tells me it’s outside of your area. (check the values being submitted for “Region”, because if I enter the details of the house next door it tells me the address is in England, not Kent).
Also on the same page “Premise”? Should be “Premises” I guess, or “Building”?
Not trying to rain on your parade, but is there a reason you didn’t just go with Google’s address lookup?