OK, so people are coming into the UK on student visas as a way to illegally immigrate.
Strikes me that there's a simple test for this: if you're coming from a country which generally has illegal immigrants then you can only come into the UK to study if the thing you are studying is outside of the norm. So, you can't come to do a fairly regular degree in computer studies, because why would you? Surely it would be cheaper to do it in your home country.
But let's say you're doing aPhd in electronics at Cambridge, or doing a course related to steel in Sheffield (which you can't do in your own country), then you get thoroughly checked before coming in.
Comments welcome on these thoughts...