There is no 'rule' in the sense that someone has created a regulation binding upon all insurance providers. However the vast majority of insurers won't provide insurance online to anyone who doesn't meet strict criteria (such as 'no unspent criminal convictions' or 'no claim within the past 5 years'). Anyone who falls outside of such criteria is either automatically rejected or has to make personal contact (usually by phone) to arrange cover.
A key criterion for such purposes is often that applicants must be under 80 years of age. Many insurers refuse to offer cover to people over 80 years of age. (That particularly applies to holiday and motoring insurance but might also extend to other types of insurance).
If I've read your post correctly, you've already got cover but you might like to bookmark this link for future reference:
http://www.ageuk.org.uk/products/insurance/car-insurance/