1080p is a greater image quality than 1080i so if you're set to 1080p then you're at the max.
In a nutshell 1080i only updates half the screen at every refresh. The next refresh then updates the other half. During fast moving images this can lead to a poor image as half your rows are from one image, and the other half are from the one before. The rows are interlaced in how they're updated (hence 1080i) - so on the first sweep rows 1, 3, 5 through to 1079 are updated. Then on the next sweep rows 2, 4, 6 through to 1080 are updated. Think of interlaced fingers.
With 1080p (or progressive) every row is updated on the same sweep. This results in a much smoother image but needs approx twice the bandwidth to do so.
As I mentioned before, Sky and Virgin currently only broadcast at 1080i so there's no benefit to 1080p via the tuners.