Would that count as forced entry as far as an insurance company is
concerned? Or would having a key 'easily' accessible mean that the
insurance company would not pay up in the case of a burglary?

When I fitted one for an elderly relative we asked. Her insurers said
that use of the key from the keybox would not count as forced entry.
Not a big deal for her as it just meant she would have to bear the £250
excess on her policy. But with other policies the effect could of
course be more painful.

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R