Language Requirement
You must prove English language competency at B1 level (intermediate). Test options: IELTS (International), Cambridge, Trinity, City & Guilds. These cost £150-300. Exemptions: educated in English-speaking country, native English speaker.
B1 Level (Intermediate)
Can understand main points in work/school situations. Can handle most travel situations. Can write about familiar topics. Can participate in conversations. Most non-native speakers can reach this with 6-12 months study.
Combined Life in UK Test
Life in UK test covers both: language comprehension (through reading the 75 questions in English) and cultural knowledge. If you pass the test, your English is sufficient. Most people take separate English test + Life in UK test.
Study Path
Timeline: Take English language test (1-3 months prep). Take Life in UK test (3-4 weeks prep). Both in same 6-month period. Then apply for citizenship with both results attached.