The largest set of publicly available real time urban data in the UK.
Urban Observatory API is a free Open Data API that requires no API key, is only available over HTTP and does not support CORS, so it should be called from a server.

Unified API suite · now live
Geocode an address, validate an email, track a flight or pull stock data with one account, one dashboard and one API key.
Official social media handles and identifiers of 155,000+ notable people and organisations
Courses, lecture videos, detailed information for courses etc. for the University of Oslo (Norway)
Yes. The Urban Observatory API is listed in the Open Data category of the public-apis directory, which only includes APIs that are free or have a free tier. Check its documentation for rate limits and paid plans.
No. The Urban Observatory API requires no authentication, so you can start making requests without signing up.
Not directly. The Urban Observatory API does not allow CORS, so call it from a server, serverless function or proxy instead of front-end JavaScript.
No. The Urban Observatory API is only available over plain HTTP, which browsers block on HTTPS sites. Call it from a server instead.