If your customer details are held on a firm's database & a competitor "hacks" them to approach you via. an unsolicited e-mail, is there any offence under Misuse of Computer Legislation? What about Duty of Care of the firm holding your details? Local coppers don't want to know!
I can't follow your question. Why should anyone hack into my customer database to send me unsolicited mail? Should 'you', in your post, read 'them'?
Anyway, the fine details are unimportant. 'Hacking' is illegal and so is sending unsolicited commercial emails (i.e. 'spamming').
The UK regulator, in respect of both hacking and spamming, is the Information Commissioner. That's where you need to direct your complaints, rather than to the police: http://www.ico.gov.uk/complaints.aspx