O2 is Be, O2 own Be. They are both the same LLU network.
There is a possibility O2 are offering a BT IPStream product to people who can't get the LLU service - just so no one's left out. So this might be why you're seeing 8Mb, and it probably means the checker's just being thick if Be say they operate there.
They don't offer a service to anyone not in an area with no Be* equipment in the exchange so I think it's just a difference in the way O2 and Be estimate how much speed you can get.
I've been using O2 broadband for about 5 weeks now and I would never think of changing it, I was pretty happy with my last broadband provider (freedom2surf) but O2's broadband support seems just as good and I've doubled my speed for a cheaper monthly price.