I'm a first year at Edinburgh, but I saw this thread on the updated feed thingy, and as we have the same 3 subject system as St Andrews I am going to comment:
Remember that you will also be obliged to take 2 additional courses to fill up your credits, so will you be less engaged, less interested, and less likely to participate in the tutorials in these subjects, in the same way as you ask about the non degree philosophy students? I'm guessing not. There are so many courses you can take as your outside- at Edinburgh its literally anything on the timetable- so I doubt anyone is really going to feel the need to 'fill the timetable' as when it comes down to it there will probably be 4/5 courses which interest you, so you pick your favourite. Its definitely not a case of 'I have a list of several courses, none of which interest me, so I'll pick the easiest', and you'll most probably have the same problem when selecting your outside subject.
In my lectures I have no idea who is taking the subject as a main degree choice or as an outside course. The only way you find out is if someone tells you. Everyone is still obliged to do the readings, contribute to tutorials, submit decent essays and generally commit to the subject, regardless of whether it is your degree or not so I really don't think you have any cause to worry. Yes, at Bristol there may only be 20 people on the course, but that’s only 20 opinions in a debate, at St Andrews you'll have loads more than that, and even if someone is completely new to philosophy it doesn't mean they can't contribute to debates in the same way that you can.
Also, I can identify with the above poster. My outside course is Spanish yet I seem to do as much work for that as I do for my other two subjects (linguistics and anthropology) combined, as it is a lot more time intensive. You literally can't get by with the attitude of 'its only my outside so I don't care' as even in first year you're expected to put the effort in and work.