Most cases of CV are mild. So, most people with CV won’t go for testing. But to add to that, even with severe symptoms, most health professionals will not be testing for CV unless there is a “reason” to believe they could have been in contact with someone from outside the UK with CV.
The result is that the vast majority of cases currently show links outside the UK, not because that is the vast majority of CV cases, but because those are the only ones that are being tested.
I’ve no doubt that many of those who are being found to have CV and that being attributed to travel abroad, actually caught it in the UK. But only those people are being found whilst the vast majority are hidden, because they aren’t allowed to get testing.