High commission of India website.
Here you can find all information about India-UK, Visa, OCI, PIO, Passport and other related information.
Keep an eye on the public notices section as well for any announcements.
Generally all you can do at High Commission in London, you can do it here too. I would suggest people to refer to the relevant site pages for full information as they can keep changing.
Emergency Mobile Contact no. for Consular Services
OCI
Further information on OCI and immigration
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Scholarship Programme for Diaspora Children
UK government site listing relevant UK-India news along with travel advice etc.