Same with prisoners being not allowed to vote.

How can you have a representative democracy where only some of the people are represented, and laws are made that determine who goes to prison for political reasons (such as using cannabis for example)?

The Brexit vote in the UK is a great example of this injustice. It is often cited as 52% voted for Brexit.

But the people most affected are the residents from EU countries other than the UK. They are of all ages, and have been told, some their entire lives, that they have the same effective status as UK citizens in virtually every way.

Unlike most systems, the EU-UK system did not require visas, documentation, or place restrictions on people. They were allowed to do virtually everything any citizen can do, and advised in effect that converting to actual UK citizenship is a pointless formality, which even the UK state would not encourage. In effect they were "virtual citizens" already. Many are part of multi-generation families in the UK by now.

And then a decision which affects them most of all is passed without their input.

There are millions of them. About 5.5% of the entire UK population. If they had been asked, there is no doubt from the numbers that the result would have gone decisively the other way.

A prime example of "taxation without representation" and all the other things like that.

On top of that, another group very much affected: UK citizens living in the EU. Some of those retired are now in difficult circumstances, as they lose rights where they are settled, and can't really come back to the UK either. Those living there long-term were not allowed to vote on the issue even though it severely affects them; that number is about 1% of the UK population.

A prime example of "screwed without representation".