I live in Europe (Germany), so not travelling there is not an option for me. However, I want to point out that you cannot deduct that because something happens in one place in Europe, it will happen at another place in Europe, too. Just like in the United States, your experience when travelling with regard to security will vary greatly if you visit Chicago or Yellowstone. In Europe, add to that there are plenty of different countries with different cultures and different governments, all that has an effect on security.
I am not sure about which attacks you read about on tourists on Med cruises. I would guess those were in Tunisia or Egypt. The Mediterranean Sea is actually bordering on to three different continents: Europe (Spain, France, Italy, Malta, Slovenia, Croatia, Bosnia and Hercegovina, Montenegro, Albania, Greece) Asia (Turkey, Syria, Lebanon, Israel) and Africa (Egypt, Libya, Tunisia, Morocco). Some of these countries, I would most certainly not travel to (Syria of course is totally out of the question), but of the European countries, I would have no problem travelling to any of those.
You mentioned a British Isles cruise and I would consider most of the cities and towns you would visit on a cruise like that relatively "safe". The only spot that could be more of a target for terrorism attack might be London. But then, in my opinion London is a very safe city, because people there have lived with the threat of terrorism attacks for such a long time. I have never been in a city where I found people to be more vigilant than Londoners.