To be honest I think the tutorial system is less isolating than other structures... I'm closer to the 6 other PPEists in my year in my college than my friends at other unis are to people on their course. Tutorials mean you are spending a concentrated hour with one person, plus discussions when you're doing your reading and writing (assuming you get on with your partner!), rather than walking into a huge lecture hall of nameless faces. Obviously that does happen with lectures, but PPEists aren't exactly renowned for attending all their lectures religiously
It's amazing how you do get to know others doing your subject but at other colleges - you chat before a lecture starts, sit next to someone on a crewdate, get introduced as a friend of a friend, go to the same society, play the same sport...
Assuming your cohort get on well in your college, you will make some strong bonds. Just moaning about the reading, letting others know which books were the best, etc. I'm not saying the other PPEists will be your best friends, but in 1st year it's good to have pleasant relations with them. We swapped all our essays between us (at the suggestion of our tutors) - you can learn so much from your colleagues. Every has a different take on a question. There may only be one other person in your tute, but there could be 6 other people who are writing the same essay (depending on how many PPEists in the college).
By 2nd and 3rd year there's a reasonable chance you'll be in tutorials with PPEists from other colleges because of options choices - widening the web further!
If you are concerned about being isolated, I'd really recommend applying to a college that takes a reasonable number of PPEists, i.e. not just 2 or 3. That way there's a higher chance you'll click with at least some of the group!