Hey there
@Erni7MTR are Samsung's Trade In partner. For myself I've found them genuine and reliable to date.
In the T&C's of your trade in, one of the key facts that you had to tick to confirm related to screen burn (I know this as I did the same with a pixel 6 pro). As they have found an issue, you do have choices:
1. Accept the revised offer and pay the £50
2. Decline the revised offer and ask them to return your trade in. You'll have to pay the full value of trade in offered to you when you checked out.
You could ask for a second opinion but from experience I've not seen posts detailing success I'm afraid.
Also not sure about your "taxed" comment, nothing to do with government.