The principle of it does go against the core ideas of how police in the UK are supposed to conduct themselves. Even if you take away the far right association.

The thin blue line suggests the police are a separate group, in-between you and some evil. This isn’t how police should think about their role.

Police officers should conduct their duties as members of the community for the benefit of everyone in the community.

Even the criminals they pursue. People who commit serious crime are often known to the police, early and constructive intervention avoids people becoming serious criminals. Crime is a slippery slope. Early intervention can turn a would be criminals life around. Police won’t see this as a possibility in their daily duties if the conceptualise their role as the thin blue line.

Policing by consent is how police forces in the UK were conceptualised initially. It’s how they should operate today. We shouldn’t import the policing practices if the USA. They are much less effective, and appear to cause harm.