How to change the suffix of my contact to a UK style?
Are you referring to the suffixes drop down, Jr. Sn etc? if so…

You have full control of the list of suffixes:

From the CiviCRM menu –

Administer > Customize Data and Screens > Dropdown Options > Individual Suffixes

/civicrm/admin/options/individual_suffix?reset=1

From here, you can add, update and edit any of the options.