I would be incredibly shocked if anyone had enough knowledge of both Warwick and Bonnoci university to give a solid answer...
If you're looking to go into IB in the UK, clearly it would be better to study in the UK - interviews are easier to attend for internships and grad jobs, which if you're going for IB odds are you'll be attending a few
^ Not to mention Warwick is one of the only 6 UK unis that are heavily considered as top target for banking jobs. You can't go wrong with Warwick if you're heading towards that route.
Definitely go to Warwick. It seems to me that Warwick has better reputation and employment prospects especially as it is a pretty good Target university.