What it means
The UK Code of Non-broadcast Advertising and Direct and Promotional Marketing, usually just called the CAP Code, covers advertising in print, online, in social posts, in emails and on your own website. Its first principle is that marketing must be legal, decent, honest and truthful, and that claims must be capable of substantiation. It is enforced by the Advertising Standards Authority.
Why it matters to a small business
It applies to a one-person business posting from the counter exactly as it applies to a supermarket, and the marketing on your own website counts. You can run a shop for years without reading a word of it and comply by instinct, which works right up until a competitor complains about a claim you cannot evidence.
The bit people get wrong
The obligation is to hold the evidence before you publish, not to produce it if challenged. Best in Kent, voted number one and clinically proven all require documentation in your hands on the day the claim goes up. If you cannot lay your hand on it, the safe move is to say the checkable thing instead.
Source: The UK advertising codes, ASA and CAP. This page is a plain-English summary written by the Marketeer team, not legal advice. Where money or the law is involved, read the source.