If ....

The rights expressed in the United Nations Universal Declaration of Human Rights are, according to its preamble, to be considered applicable to everyone and they are to be considered as "inalienable". This latter descriptor means that the rights enshrined in the Declaration are not to be taken away from anyone for any reason. Political opinion is explicitly referred to as one of the distinctions that do not allow for derogation from the rights expressed in the Declaration (cf Article 2).

Article 12

No one shall be subjected to arbitrary interference with his privacy, family, home or correspondence, nor to attacks upon his honour and reputation. Everyone has the right to the protection of the law against such interference or attacks.

It seems to me that, rather than being a question of freedom of expression (cf Article 19), the debate about the playing of a certain song by the BBC is a question relating to Article 12. That is, it is  question about attacking the honour and reputation of an individual. The question facing the BBC is not one of restricting or not restricting freedom of expression, but one of whether or not they wish to make themselves party to a breach of Article 12. Freedom of expression can be adequately achieved by observations of political difference from Mrs Thatcher.

However, both during her own political career and since, it has been socially/culturally acceptable to attack her reputation. Now, if, instead of being Mrs Thatcher, the person being subjected to such attack were ..[substitute here your own particular favoured figure] .. or if that person today, say, were to be well known in the LGBT community, then outrage might follow. Why not for Mrs Thatcher?

The right at stake is universal and inalienable - so Mrs T is entitled to it as much as anyone else.