The Post Office is still the dominant UK postal service despite having its monopoly ended in 2006. Should you need to reach out to its customer service department, this is where you can find contact details.
Contact the Post Office by telephone
Got a query, want to give feedback or make a complaint, you could trying ringing the customer support telephone number 0330 or 0345: Calls to 03 numbers cost no more than standard geographic numbers starting with 01 or 02 from your fixed line or mobile and may be included in your call package dependent on your service provider.
Alternative ways to contact the Post Office
The Post Office customer services pages Post Office should be able to provide you with ways of getting a solution. There are a string of FAQ sectors, each with dropdown menus, some leading to online forms. If you want to find the nearest post office, the Help page has a branch finder service.