A person can remain with ILR staus for the rest of their life and as long as the rules/laws don't change drastically can apply at any time.
Many Indian couples the husband would become a british citzen and the wife remain indian. As in the past India did not allow dual citzenship and it allowed them to keep a foot in both doors. As many of the ones i know either were kicked out of africa in the 70's or knew of people who had so realised the importance of keeping your options open.