There are several organisations and businesses providing professional people search services for the UK. These range from the charitable and not for profit organisations through to the businesses that specialise in training and locating people. Selecting one that meets your requirements will depend on the nature of your search.
For example, the Red Cross is commonly used when attempting to locate a family member internationally where there was a separation caused by a conflict or natural disaster. Whereas if you are looking to locate someone to recover a debt, or serve a summons, a business option is the only real choice.
Whilst there are an excellent range of options to assist in locating someone, there are no guarantees and it can be time consuming. Experts spend their days involved in the tracing of people through a range of avenues. They have developed experience and knowledge that can be useful if not essential with a search.
The approach you choose may come down to the time and resources you have at your disposal.