No, you do not sound like a complete idiot at all. It sounds like you were sent the wrong email and the submission link was for US based organizations.

You should be able to submit your UK account here: http://services.google.com/googlegrants/accountcreation?hl=en_GB

One thing, however; You had mentioned about "inputting payment details as requested". DO NOT, put in any payment information. This will turn your grant account into a paid account. Please follow the steps in the Account Creation Guide VERY carefully or your account will not be approved.

You can download a copy of the UK account creation guide here: http://static.googleusercontent.com/external_content/untrusted_dlcp/www.google.com/en/us/intl/en_uk/grants/education/Account_Creation_Guide.pdf

All the best,

Robert