Just do not think about it. Seriously. I almost canceled my very first trip (Europe-SFO with a connection in Germany) because I was scared of the 11 hours flight. A friend had to talk me out of calling Lufthansa to cancel. Once on the plane, I pretty much got resigned that either things will go well or not. Fast forward 10 years or so and I rarely have an year with less than 100K miles flown...

It's a plane. Bigger than the ones you are used on domestic but still a plane (bigger helps actually). Don't worry and enjoy your flight and your time in Europe.

As for London's high alert - they do this all the time - for various reasons. I am surprised almost any time when they are not on at least orange. So... don't get worried because of that. I used to joke that London goes on high alert when there is a chance of rain (which is a few times per day)

And as someone else said - stop watching the news or stop giving them so much credit. It is the holidays - of course people that have nothing else to report will start talking such nonsenses.