Finding for 8 is exceptionally difficult. It's frustrating, but with larger houses people do need to decide who they want to live with very early, far earlier than a couple of years ago it seems. We've had 3 groups have a look round our 7 bed house so far, and we haven't even yet confirmed to the estate agent that we're not retaining it next year. To put that into perspective, when we were househunting for 7 two years ago, we hadn't even decided to live together until the first week of second term.
If there is a realistic way of breaking into two groups of 4 and looking for houses very close, if not next to, each other, you should definitely consider it. As HD said, the majority of large houses have been broken up in some way. Having seen probably most 7 bed houses in Leamington I'm not exaggerating when I say ours is probably the only one with no box room, two bathrooms and a living room. With 8, it will be even worse. They do exist, but as the number of housemates goes up beyond 5 the amount of time you have to work with goes down.