The part that you don't understand is that Ukraine is fighting for it's own survival, against a genocidal enemy.
This war is not just a matter of lines in the map. The lives of million of people are at risk.
Not only this is not the first time Russia invades Ukraine, but every time that Russia controls it, life turns very bad for the Ukrainians.
For example, the Holodomor was a famine caused by the Russians in 1932, as they stole all the grain from Ukraine. The result was the death of up to 5 million Ukrainians.
And even those who survived, suffered immensely because of this.
There is a reason why the countries that are giving most aid, in percentage of GDP, are countries like Poland, Lithuania, Finland, Estonia and Lituania.
Because they have also suffered a lot under Russian rule. And they know that if Ukraine falls, they will be next.
If the UK was to ever face an enemy as cruel ad sadistic as Russia, you can be sure that you and your family would suffer. Immensely.
EDIT:
Have you ever heard about the The Soviet War Memorial, in Treptower Park., Germany. It was built right after WW2.
The Russians call it the "Tomb of the Unknown Soldier." German Women call it the "Tomb of the Unknown Rapist"
Take a guess at what would happen to your wife and kids, if ever the UK, was to face Russia.