One is an international standards body, the other is a regional (part of Europe) standards body. Just like China's national standards and local standards, one is implemented nationwide, the other is implemented within the province. But IEC and EN are both more influential. Many countries consider and adopt, even equivalent, when setting standards and exporting. However, IEC and EN are recommended standards, not mandatory standards. International practice, mandatory called regulations, directives, etc. (generally product standards, or related to hygiene, safety, health, etc.). Foreign standards are for the regulation, directive service, supporting. Unlike my country, regulations are administrative in nature and there are few technical regulations, which are issued by mandatory national standards.
The reason why the two give a similar impression is that they, including ISO, often refer to each other and convert. As we say, the past serves the present and the foreign serves the present. In fact, international and regional standards organizations will also adopt standards of other countries if they think they are good. For example, the famous ISO 9000 originated from the United States military quality assurance standard MIL-Q-9858.
The reason why international standards and regional standards often adopt each other's standards is that as transnational standards, their universality must be fully considered in the formulation, that is, the interests of member countries (also the requirements of WTO). So international standards are not necessarily the most advanced. Two EU directives, such as the one that made headlines recently, set an agenda to create barriers to trade while keeping member states' interests in mind. But many member states are also behind schedule.
BS EN is not EN. It is technically the British standard adopted from EN by BSI. It's just that it belongs to the equivalent adoption. And in recent years, the emphasis on intellectual property rights has been put on all but the first few pages, followed by the original EN (with BSI's own markings on the side). It wasn't always like this. pr EN seems to be a discussion paper, not a formal version. Some pr EN have no further discussion, and have never been formally promulgated. The reason why IEC promulgates in English and French is, I guess, because English is the most widely used language in the world at present, but the rigor of technical work must be regulated in French, the most rigorous language in the world. In case of any discrepancy in understanding, French shall prevail. Each ISO standard is also published simultaneously in English, French and Spanish, but unlike IEC, which combines English and French, each language appears to be a separate book.
At present, there are several kinds of common EN: EN (EU issued), BS EN (British transformed), DIN EN (German transformed), NF EN (French transformed), but because of the structure of most people's foreign language, the most common at present is BS EN.