This Finpip overview summarizes City Index as commonly marketed to retail traders (founded 1983; HQ: London, United Kingdom). Typical platforms include Web platform, MetaTrader 4.
Regulatory snapshot (not legal advice): FCA; ASIC; MAS; StoneX group; CFD and spread betting (UK)
Official website: https://www.cityindex.com/. Confirm licence numbers, product availability, and costs there before funding an account.