Independent schools:
Schools want to ensure there is sufficient English language competence for students to get the most out of their lessons, their friendships and as a basis to solidly improve their language & academic skills.
However, there are schools featured on the site that offer international study centres that cater for students with a lower level of English competency…

Universities:
Typically, 6.5 IELTS is the language standard at which universities will accept international students but there are increasingly exceptions to this and some Universities offer a lower entry level.
It is strongly advised that students do have an overall 6.5 IELTS level before applying to a UK University to ensure they can fully access and understand their chosen course from day 1 of their studies.

This published article, written by UK Education Guide, explains more about the importance of English competency before commencing degree study in the UK.
https://studytravel.network/magazine/news/2/30658