Hi, and welcome.
If you marry an Indian citizen you can apply for Indian citizenship after you've been ordinarily resident in India for seven years.
The UK doesn't have any ban on dual nationality - for instance, both Kay (this forum's owner) and I are dual British/Irish nationals. So if you choose to acquire Indian citizenship, you don't automatically lose British citizenship - if you were born with it, the only way you can lose it is to renounce it by making a written declaration to the Home Secretary.
However, several countries do forbid dual nationality for their citizens, and India is one of them. I doubt whether they would go so far as to demand to see evidence that you have formally renounced it before they grant you citizenship, although you might be required to swear an affidavit or make a solemn declaration as part of the citizenship process. But I'm sure they would take a dim view of any attempt by an Indian citizen to enter India using a British passport.