Immigrants applying for permanent settlement in the UK must pass a 'Life in the UK' test which demonstrates their understanding of English language and culture. If an immigrant's English isn't up to taking the test, they must take ESOL (English for Speakers of other Languages) lessons and advance one level, obtaining a certificate as verification. These lessons are expensive. This applies even to spouses who have married a UK citizen. Many of the terrorists apprehended so far were born here in UK, but went to train in their chosen career as murderers in fundamentalist states. They're not in any way immigrants.