To be able to become a new British citizen, you need at least to have English skills on the B1 level. So, you need to prove that if you want to obtain your new status as the citizen of the country.
The best way to find out your CEFR level is to take a well-designed standard test and the test that you need to take is the B1 English test that you can book on britishlifeskills.com. In English, EF SET is the best choice as it is available for free online and is the first test aligned with CEFR. You need to spend 50 minutes to complete the test and know your CEFR level.
To find your CEFR level in other European languages, all of the most common assessment tests have been aligned with CEFR. You need to take a different test depending on the language. Check with official institutions for languages in Europe, such as Alliance Française for French, Instituto Cervantes for Spanish, or Goethe Institute for German.