Its not dual citizenship, you can apply for citizenship if your married to a Brit, I don't know the how long but its straight forward, loads of lawyers outthere thats there main income, not overly expensive, you can go to citizens advice buroeu for free to fill in the paperwork.
For Indians for example you can apply for a OCI card which is gives foreign people of Indian heritage to avoid applying for visa's. No need for dual citizenship.
Majority of Indians and Pakistanis first gen are dual citizens.