What English level do you need for the UK High Potential Individual visa?

Many HPI applicants hold a degree taught in English. Gov.uk accepts a degree-level qualification taught in English and awarded by a UK institution, or a degree from outside the UK if Ecctis confirms it is equivalent to a UK bachelor's degree or higher and was taught in English.

Note the separate Ecctis check for HPI itself: your qualification must be at UK bachelor's, postgraduate or PhD level, awarded in the last 5 years by a university on the eligible list for that year, and Ecctis "will not check if your university is eligible" (gov.uk).

Nationals of Antigua and Barbuda, Australia, the Bahamas, Barbados, Belize, the British overseas territories, Canada, Dominica, Grenada, Guyana, Jamaica, Malta, New Zealand, St Kitts and Nevis, St Lucia, St Vincent and the Grenadines, Trinidad and Tobago and the USA do not need to prove their English. You may also not need to prove it again if you did so in a previous successful visa application (gov.uk).

You can only apply for an HPI visa once, UK universities are not eligible, and you cannot apply if you have already had a Graduate visa (gov.uk). Rules change, sometimes at short notice. Confirm the current requirement on the gov.uk HPI page before you book a test or apply. This guide restates the official page; it is not legal or immigration advice.