Upvote:2
I did not provide National ID (Aadhar), instead I filled my passport number there and it was accepted. I got my visa without any issues.
Note that its not the universal case. According to Indian government, Aadhar is still not technically mandatory. So, UK govenment can't just reject the visa citing I don't have an Aadhar card. I was an NRI when I got the visa. Aadhar compliance is lower among NRIs. That could also be the reason. (For example, NRIC/FIN is mandatory for a Singapore resident/citizen. If you are a Singaporean, you have to provide NRIC without any exceptions).
If you have an Aadhar, its better to add that.