I have an Iphone 3GS os4.0.1. It is jailbroken too!

I use a site to stream movies to watch and can do successfully whilst on my home wifi internet but when i try to watch whilst on 3G at work it will give me a error message 'this movie is not supported'

I have tried 3G restrictors and downgrading back to os3 but to no avail.

I am in the UK so am on the O2 network.. I have tried a friends iphone who is also on O2 and he has the same problem but i asked a friend who is on Orange and his worked fine.

Has O2 somehow managed to restrict me streaming large files??

Any help would be most appreciated or any ideas..

Thank you....