The history/politics focused personal statement may be a problem for St Andrews, yes. How high up is St Andrews in your university choices/priorities.
The difference between St Andrews, Warwick and Durham in terms of general "rep" is equal. In International Relations/politics St Andrews would have the edge. Certainly in IR anyway. Durham and Warwick are equal imo.
Of course rep is based largely on general perceptions. It's quite a hard thing to measure and slightly subjective. Still, I don't think too many would disagree with me there.
Although I see you have chosen Durham's European Studies as an option. Why not History and politics? I think the joint honours programme has now been discontinued. However, you can still create yourself a far more flexible joint honours programme through the combined honours programme. I only say this as it seems you are drawn to history and politics elsewhere, Durham's history department is excellent (arguably the strongest in the country, certainly outside of Oxbridge) and your PS is focused on politics and history.
As I've said a few times, I don't mean to give the impression Durham's politics department/school of government and international affirs is a bad department at all. It's certainly not. It has great strengths in political thought and middle eastern politics in particular. I enjoyed my time there and would still recommend it. Even so, I would be lying if I told you its politics department was of the same standard as its many of its arts and science courses (for which it's so well regarded). History, English, Classics, Physics, Chemistry and law for example. The only minor concern I have with politics at Durham is the school's slight lack of organisation. It doesn't cause any major problems though.
But if European Studies appeals to you, then, fair enough

I just wasn't sure if you were aware of the combined honours.
Finally, Durham and St Andrews are fairly equal in terms of general cost. I'm not 100% sure about St Andrews' accomodation but I've heard it's quite expensive. Durham's is fairly expensive also. For first years it's not too bad, for returned it's ridiculous. However, you don't need to live in as a returner. You can chose to live in private accomodation (the cost of which varies significantly but if you're quick of the mark you can get a good deal). Other than accomodation then other costs are reasonable.
Sorry I can't say much about Warwick. I'd rather leave that to Warwick students.