I hope UNIT doesn't play too big a part in the special. I haven't really cared for the other, UNIT-central episodes in the new series all that much.
Yeah, thinkin about it... The Christmas Invasion is good, but they're more of a side thing in that. The first major one to feature UNIT is the Sontaran two-parter, which is pretty much a disaster. After that, Planet of the Dead, another disaster. The Power of Three, some nice character scenes; mostly disaster.
But I think this will just be a small thing to get the Brig's role fulfilled.