I’m a Bristolian who has firmed Exeter for law so I can try and help you out with this.
Like you’ve said, Bristol is a fantastic city and a vibrant place to be a student — plenty to do and some of the scenery where the uni is situated is something to behold. Great law course as well and Bristol is highly reputable; you can’t really go wrong if you choose it. However, as you’d prefer to be on a campus, Bristol is very spread out — the Stoke Bishop accommodation say is miles away from the Wills Building and the centre.
Exeter on the other hand is a beautiful campus in a really nice location (hence why I’ve firmed it) — it will be quieter than Bristol and less to do but there will be likeminded students there and I doubt you’ll be bored. Again, it’s a reputable university with a very good course and from purely a reputation perspective, there’s not a great deal in choosing between Bristol and Exeter.
I’ve not researched Lancaster a lot if I’m honest but from what I’ve heard it is very isolated like you said, which is something which would put me off hugely. Don’t know if it’s quite as good as the other two either.
I’d narrow it down to Bristol and Exeter and make it dependent on whether you want a city university with a vibrant student life or a more quaint campus and try and base it on that as they’re pretty different.