It depends on the university. Most don't have a provision for international part-time students, so you get home fees, but apparently some are now closing this loophole. I'm on a spousal visa as well, and I go to Oxford Brookes part-time on home fees.
I hate that I can't go full-time though. I assure you, I did not come to the UK just to take advantage of the universities here! Seems like if you are here as a spouse they ought to give you a break.