It's illegal, under the Data Protection Act, to provide 'reverse searchable' directories (whether on paper, on CD or online) within the UK.
When phone directories first became available on CD, some directories offered this facility. However, many people expressed security concerns about this. (An example which was often quoted was that of a young woman who'd just met a guy she likes at a nightclub. She'd be happy to give him her phone number but might want to keep her address secret).
Occasionally you can do a reverse search by simply putting the number into Google. (Remember to enclose it in double speech marks so that Google searches for the entire number, including the STD code). However, this will only work if the number belongs to a business or to a person who has put their personal details online (e.g. the secretary of a local club).
Otherwise (apart from bribing a BT employee to break the law) the only way to find out who a number belongs to, is to phone the number.
Chris