Mrs T was forced to get rid of the MMB by EU completion rules.
Why did the Royal Mail need privatising? It was turning a profit and bringing money in for the government and providing a good service. it was privatised to satisfy EU anti monopoly laws, applied absurdly and in extremis to what should and has to be be a universal national service not a private business which will ultimately end up cherry picking the most profitable services and leaving remote rural communities with a very much degraded service.
Royal Mail in common with so many other national mail operators had their whole business model usurped by the digital age. Blaming Mrs. T or EU or any other entity is just naff!