There's a bit more to it to be fair that makes a fair bit worse. They're not just violating England's camp rules and quarantine, they are also violating Iceland's rules around quarantining and therefore risking it spreading either to the England camp or in to Iceland if they are carriers and unaware. Also aside from the general global pandemic stuff, Foden has a longtime girlfriend and a kid at home, which makes it that bit extra scummy.
They're both idiots simply.