Cambridge IGCSE First Language English (Syllabus 0500 and 0522) and Cambridge IGCSE English as a Second Language (Syllabus 0510 and 0511) are no longer included on the UK Visa and Immigration's (UKVI) approved list of English language tests for immigration purposes (SELT list). This means you cannot use them as evidence of your English ability when applying for settlement and naturalisation. UKVI continued to accept Cambridge IGCSE English Language qualifications as approved English language tests until 6 April 2014. However, after this date, the qualifications are no longer accepted as approved tests by the UKVI.
Visit the UKVI website at https://www.gov.uk/browse/visas-immigration for more information.