Essel
if you tell us what aircraft you are asking about someone will have the answer.
I think SNS3 covered it very well as he always does.
What is safe and what is legal.
In the UK there is no requirement to have an anti-collision beacon fitted. But if you do have one fitted it must work.
In day VFR working or not it is quite useless but would be illegal to take off with it inop.
I suspect that if someone approached a pilot after a flight and informed him that he was flying illegally because the ant-coll did not work his reply would be
It did on the preflight 
Edit
Just had a thought, if fitted it must work, but I am not sure that you are legally required to turn it on.