yes because the matches were all-over europe so same people who were in a packed pub a few days earlier were now on a plane to some other european country to watch the next match.

further, the borders remained open so people mixed at home and then went on holiday/visited family etc. UK is a major international transport hub and also has a large population of Indian descent so we likely "imported" delta from India relatively early as a result and then likely helped seed it in other western nations.

They should have put India on the red list several weeks earlier than they did when it was obvious for anyone to see what was going on, but they delayed for unknown reasons