That doesn't explain why there are protests in the UK against the actions of a country a thousand miles away, when other countries do, and have done, far worse yet get no criticism at all.
(Israel isn't the world's most high tech military, BTW)
It is also because Israel is an ally.
I believe the disgust in Britain is down to the confluence of circumstances I have mentioned. I can't think of a similar example in my life time. Maybe the siege of Sarajevo. Imagine if the USA blockaded Cuba to stop anybody escaping and then carpet bombed the island continuously for two years. I think that would garner a similar reaction. I have always been pro-Israel and was sympathetic at the start. But they have lost a lot of support, including from huge numbers of Jews worldwide.

