I understand that there's a stupidity check - anyone too dim to understand the term 'self
declaration' or to follow the CAA website guidance should probably find another hobby, preferably something 2-dimensional.....
OK- the CAA won't issue you with anything. Assuming that your declaration wasn't bounced back, just print off a copy of the form (keep a note of the tracking number), keep it with your licence and go flying. Have fun!!