The PIE News has some useful numbers regarding which tests were used by students entering the UK for studies in the 23/24 enrollment cycle. The numbers come from Enroly, which apparently processes a third of all international student applications for the UK.
It reports that about 65% of students took an English to show they met the language requirements for a student visa.
About 34 percent of students used an IELTS score to meet those requirements. About 8 percent used a PTE score. About 10 percent used a score from one of the other SELT tests. About 13 percent used a score from a non-SELT test.
TOEFL and Duolingo are the biggest non-SELT tests, but there are others.