A B1 level of English would be sufficient for interactions with English speakers on familiar topics.
In the workplace, people at a B1 level of English are able to read simple reports on familiar topics and write simple e-mails on subjects in their field. However, a B1 level is not adequate to function fully in the workplace in English.
A B2 level of English would allow you to function in the workplace in English, and indeed, many non-native English speakers in international workplaces have this level of English.
A person working in English at a B2 level will, however, lack nuance particularly outside his own field. He may also miss some of the subtleties and implied meanings in conversation.
A C1 level of English allows for a full range of functionality at work or in an academic setting. The C1 level would allow for full autonomy in a native English-speaking country.
Can understand the main points of clear standard input on familiar matters regularly encountered in work, school, leisure, etc.
Can deal with most situations likely to arise whilst travelling
Can produce simple connected text on topics which are familiar or of personal interest.
Can describe experiences and events, dreams, hopes and ambitions and briefly give reasons and explanations for opinions and plans.
Can understand the main ideas of complex text on both concrete and abstract topics, including technical discussions in his/her field of specialization.
Can interact with a degree of fluency and spontaneity that makes regular interaction with native speakers quite possible without strain for either party.
Can produce clear, detailed text on a wide range of subjects and explain a viewpoint on a topical issue giving the advantages and disadvantages of various options.
Can understand a wide range of demanding, longer texts, and recognize implicit meaning.
Can express ideas fluently and spontaneously without much obvious searching for expressions.
Can use language flexibly and effectively for social, academic and professional purposes.
Can produce clear, well-structured, detailed text on complex subjects, showing controlled use of organizational patterns, connectors and cohesive devices.