so you'd hope that assets would have already been identified or counter measures put in place to an extent
Like what assets? Between Blair, Brown and Cameron our government dismantled the existing Nimrod maritime patrol capability and the Cameron cancelled its replacement. We'll be relying on the handful of the now rather old Type 23 frigates, and the rather binary threat of our handful of hunter-killer submarines.
If a Russian plane flew into our airspace, I wonder if it would be shot down
Very, very unlikely, although it depends how much of a diplomatic incident you want. The more belligerent nations (eg Turkey) are happy to take the consequences, but between the major powers they do try to avoid getting into that sort of situation. Since UK territorial water are only 12 miles from the coast, most of the QE sea trials will be in international waters anyway.