If you are going to sell door to door only, then you need a pedlars certificate which is issued by the police.
A pedlars certificate is a certificate that allows a person to sell goods (usually trinkets, household goods and other hand made objects) door to door going from place to place. The "pedlar" cannot remain in one place and sell items; a pedlar goes to their customers to sell items rather than allow customers to come to them. If you do want to remain in one place, on a street for example then you would require a street trader's licence, which you obtain from your local council (details below).
A pedlars certificate is obtained from the chief of police from the area where you live. You need to attend at your local police station with two forms of identification, one with a photograph and the other showing an address where you have lived at for at least 28 days within the district you are applying for the licence. You must be aged over 17 to apply for a certificate.
The police need this information to check your identity and that you are of good character before issuing a certificate. The cost of the licence is currently £12.25.
Please note that having a pedlars certificate is in no way an indication that the goods for sale are of merchantable quality.