From the BBC:
A man has been warned after he dialled 999 to complain about a prostitute's looks after meeting her. West Midlands Police said they were contacted by the caller who said he "wished to report her for breaching the Sale of Goods Act"...
Despite the man refusing to give his details, police were able to identify him and have sent him a letter warning him about his actions. The Sale of Goods Act 1979 gives consumers legal rights, stipulating goods which are sold must be of satisfactory quality, be fit for purpose and must match the seller's description.
Duh.
Not only could he have been done for 'soliciting for sex' and 'wasting police time' (as the article says), but his first big mistake was referring to the Sale of Goods Act 1979. A prostitute provides services so the relevant legislation is the Supply of Goods and Services Act 1982.
His second big mistake is not realising that there is a world of difference between criminal offences (which are a police matter) and purely contractual or civil matters (where you have to take the person to court yourself).