iGCSEs are the same exam boards as GCSEs one is not more widely regarded then the other , they are just on two different levels, iGCSEs are cheaper to run and easier to pass, hence them being banned in state schools.
Why do you think state schools are not allowed to use them as GCSEs? if they were the full equivalent, they would be an option, wouldn't they. They are not, so they are not.
This is a universally known fact.
The only people I have ever come across who don't know this are on this thread.
What do you THINK the difference is? Why would any exam board make a different GCSE for pupils taking an English qualification abroad? because poorer schools abroad don't have the resources - internet, science lab, library, sports equipment, DT workshop etc that uk schools have.
Even rich schools in other countries might not have what is expected in a UK school, as the culture is different, so an english qualification might not be possible, in schools where the culture is not to have science labs, for example. Even many European countries teach science in theory only
The iGCSE is designed for that purpose. What do you think they were designed for?