If you're interested in both modern and ancient architecture, a visit to Austria and/or Germany could be a good idea. They have very innovative buildings, some beautiful some hmmm depends on you...
But as of UK and Italy, beside a 6-hour visit in London, I haven't laid a foot on yet... There are plenty of tpunks who know the countries very well, you'll get more answers soon. In the meantime, here are a couple of related topics you can find in the "Where you all going" forum, under "Europe : Western".
I invite you to have a look at the whole forum, if you haven't yet !