You're wrong, get a clue:
"If you’re travelling to England for less than 10 days
If you’re travelling to England for less than 10 days, you will need to quarantine for the whole of your stay. You must travel directly to your place of quarantine when you arrive in England and directly from your place of quarantine to the port or airport when you leave. You should follow safer travel guidance and avoid public transport if possible.
"
Check if there are any COVID-19 rules in place for travel to England from abroad.
www.gov.uk
They'd quarantine for the duration of their stay, but they wouldn't have to stay quarantining in the UK for ten days. They could be in the UK for 2 days and then travel back to their amber country. Whether or not that would mean they could attend the game is another thing though.
I digress though, UEFA are scum.