E-cigarettes are electronic products that use batteries and electric heating elements to convert liquid liquid into gas. Because of its similarity to real cigarettes in appearance and use, it is banned or severely restricted in countries such as China, Australia, and New Zealand.
If you're planning to fly from the UK to India then you can take e-cigarettes with you on the flight, just make sure you don't smoke them in public. In addition, you also need to pay attention to whether it violates local laws and regulations, such as whether electronic cigarettes are allowed to enter the country and so on.
In short, if you plan to take an international flight, please be sure to understand and abide by the laws and regulations of the destination country in advance to avoid violating relevant regulations or causing unnecessary troubles.