OP isn't just saying that crime is unreported. Because if all crime everywhere is unreported we can still compare Coventry with other towns, and when we do that we see that coventry has a lower than average crime rate. OP is claiming that Coventry is unique in having under reported crime and that everywhere else is accurate.

But not only that. There are other statistics - the british crime survey - (and these are robust statistics) that do not rely on crime reported to police. And OP is again claiming that Coventry is unique in reporting very low rates of crime while everywhere else reports accurate rates of crime.

But even if we assume that's true - that Coventry is unique in reporting and surveying low rates of crime while everywhere else is accurate - there are other indicators of crime. We'd expect to see higher rates for car insurace, and we don't. We'd expect to see higher rates of home contents insurance, and we don't.

OP is subject to a pretty obvious confirmation bias. "I had my bicycles stolen, and this i know Coventry is high in crime", and now they reject anything that points the other way. When you ask OP about the evidence they just come back with anecdote, when you show OP anything that contradicts that they say it's wrong (not just wrong, but wrong in very particular ways).