1. There's no 'law' that says your current ISP must give you the MAC (Migration Authorisation Code) number when you ask for it. The SKY customer service representative gave you false information on that aspect of things. The MAC process is part of a voluntary industry initiative. This means that ISPs who make it difficult for their customers to obtain a MAC are unlikely to be in breach of any formal obligations.
2. Most ISPs are pretty good when it comes to providing a MAC number.
3. OFCOM is currently looking to introduce new regulations that will apply to all ISPs which will make it mandatory to supply customers with MAC numbers on request and to comply with a specific process for doing so. OFCOM will also work with the industry to develop a process for customers to obtain a MAC from another source if their own broadband provider is unable or unwilling to comply with a request.
I suggest that you do what your ISP has suggested, and write to them about this. Make it clear that you wis to receive a MAC number without further delay.