In your original post, you seemed to be concerned that you're impression of Durham University as "prestigious" was potentially misguided, at least with regards to its business school. Now your concern seems to be its student satisfaction rating.
Do you have offers from Warwick, Bath or Exeter? If you do, then from what you say ("are known for their business schools and rank higher than Durham for business/management, with higher student satisfaction rates and even higher average salaries after graduation"), then you could accept one of those offers over that from Durham. If you did not receive offers from Warwick, Bath or Exeter - and only have Durham to choose from - then your choice is easy.
If you're still waiting to hear back from Warwick, Bath and Exeter, then it makes sense to wait until you do before trying to decide which (theoretical) offer you would be better accepting.
I can't explain why your offer would be below the standard offer, other than to point out that as an international student you are more valuable (in terms of fees) to the university than a home student would be. I've no idea whether that has any impact of not, but it's the only reason I can think of for the lower-than-expected offer.