Australian Native English Speaker Studying In UK?

An acquaintance, borne in Australia, speaks nothing but English, and has done all their schooling English is thinking of studying in the UK. Yet I keep reading you need to demonstrate English proficiency to study in the UK. I know for a UK student to study in Australia all that's required is they show all their schooling was done in English. I am perplexed - do you have to show English proficiency by IELTS or similar. If so it sounds absurd. The person concerned is also going to study some subjects via the University of Open Learning n the UK. You can do that in Australia - take a reduced number of subject (4 instead of the normal 6) and do some study at a tertiary institution. If passing that does not demonstrate English proficiency as well as the other factors I mentioned it makes no sense at all. So what is the actual requirements in such a situation. BTW they are not wanting to go to Oxford or Cambridge - just a reputable university like the University Of Manchester. A big attraction for them is you get an honors degree in 3 years - it takes 4 years in Australia and you can do a Masters in many programs with just one extra year.