Is a law still a good law if nobody actually follows it?

This depends on how we define the concept of "law", which academic scholars have been at war over for decades. Some people take what's called a teleological point of view, where the procedure that goes into actually making a law (e.g. Parliament, Royal Assent, precedent) is what makes it a "law" in the first place. Some go beyond that to say that this is redundant if it's not effective - this is what we call consequentialist. There is no wrong or right answer to this question, but variations of it are very popular in Oxbridge interviews because it demonstrates how an applicant thinks about the role of the law in society - whether it's merely formalistic or whether it actually has to achieve a purpose to be good law.