In the United Kingdom
The government agency responsible for enforcing consumer protection laws in the UK is the Office of Fair Trading.
Each country in the EU has its own consumer protection legislation. It also has its own self-regulation system.
In Europe the E-Commerce Directive attempts to simplify the jurisdictional issues by introducing the “country of origin principle.” This means that a company in the UK only needs to comply with UK laws on advertising and may disregard the laws of other EU Member States, even if selling to those Member States. However, there are important exceptions to the country of origin principle, notably the terms of any consumer contracts and attempts at harmonisation are restricted in their effect. The major part of each country’s advertising regime remains nationally based but there are major differences between the regimes in force in various countries. For example, Germany bans certain forms of promotional activity (such as two-for-the price-of-one offers); Spain bans adverts for ‘war’ toys in certain media and Denmark is particularly strict in the rules as to adverts directed at children
In the USA
In the United States online advertising is governed and enforced by the Federal Trade Commission (FTC). So a UK company selling to the US should also comply with the US rules on advertising.
When trading beyond the EU, the country of origin principle has no effect.