You were tested in order to receive your immigration visa - knowledge of English is worth about 5 points (if I remember correctly) of the approximately 120 points (which changes each year) to qualify. Your test would have been simply to get the five points or not, if you had failed, you probably would have had enough points for the visa anyway.

However this would not have affected your eligibility for citizenship, as the only requirement is to fill in the necessary paperwork, turn up at a ceremony, have coffee and biscuits and go home. I know because I did exactly that in 1983, and I have friends who have done it in the last few years as well.

The last time I checked the UK also had no tests as a pre-requisite to citizenship, there was talk of introducing one, but I doubt this went anywhere.

Ooops - correction on this one - it looks like you are assessed for "basic" English, unless you are over the age 50, in which case you are not. Quite a few of my school friends had parents that didn't speak English, or very little, so they must have managed somehow. I don't recall an interview though, so maybe being a native speaker enabled me to skip that part.