It is not secured and computer does not have a unique login for every member of staff.
They haven't a leg to stand on and if you had a decent lawyer or even union rep they would be sent packing with their tales between there legs.
I also suspect that it in breach of civil service guidelines for internet access as in theory every employer should be able to tell the police who was logged in where if there is any issues with illegal activity.
just stick to refusing to attend until they prove that you sent them. Until they do there is no charges to defend.