The high tariff for entry on the Bristol course is because of the level of competition. The competition comes from many people wanting to be at Bristol because of the course, the reputation of the university, what the university offers and the desirability of being in Bristol. The latter is a huge pull, a lot of students really like Bristol as a city, many more than like Southampton as a place.
The larger the number of people apply, the harder it is going to be to get in. However for all this talk of 'higher calibre of students', bear in mind that Bristol makes a lot of contextual offers and these contextual offers are less competitive. On the politics and IR course at Bristol, the standard offer is AAA, the contextual offer is ABB.
My DD wants to study International Relations and Politics, but doesn't want to go to Bristol so it's not one we're looking at. We did go to the Southampton open day the other week and went to the politics & IR talk. DD liked the course and we both found the talk really engaging. At the subject stand came across another related subject that she was even more interested in, and we spoke to the staff and it seemed a good fit for DD.
DD liked the Southampton campus, although not as much as a few others we've seen. She didn't particularly take to the town but that was of less priority for her. I think she's likely to put it down as one of her five choices, although not at the top, because of the content of the course.