If you are looking for alternatives for Duolingo, any of the following tests can be taken, as those universities accepting Duolingo will be accepting any of the following tests too.
IELTS
TOEFL – minimum 90
PTE – minimum 50 - 60
Cambridge English test – minimum 200
According to your choice and convenience, you can take any of the tests. Make sure you follow the guidelines and prepare well according to the syllabus.
As Duolingo test gains more acceptance in the UK, you can choose the test to prove your English skills. It can be taken anytime you like, whenever you are ready. Since it is affordable, you can go forward with the test for your undergraduate or postgraduate dreams in the UK.