An employer has a duty of care to its employees to ensure your health safety and welfare. Part of this involves taking steps to minimise risks to health. That's the main statement under which you should discuss this problem with your manager. You have the right not to be disciplined for challenging what you see as an issue under the Health & Safety at Work Act, but it doesn't extend to refusing to come to work.
Have you discussed it? What has been done, if anything?
The dust from the droppings is the hazard here. Maybe a wet & dry vacuum cleaner with a bit of water inside.
Should also get rid of the mice from inside buildings.
Not as bad as leptospirosis / weils disease from rat pee in wild rats.