Some Indians speak English fluently, but in a way that most British people find incomprehensible. In contrast, and somewhat ironically, a German speaking bad English is often easier to understand than a random British person (as opposed to a British person from one's own social class and locality).
The level of English knowledge you need to study maths or engineering at a British university is obviously very much lower than the level required for studying law or working as a lawyer. Some otherwise intelligent people are bad at languages and will realistically never achieve knowledge of another language to a level sufficient to successfully work as a lawyer. In fact, I have been reliably informed that some Germans give up on physics as a career because they realise that they will never manage to learn English well enough to succeed in academia, which is a bit sad, really.