theoritically it should be the adjective related to United Kingdom. Sometimes an English person is referred to as an Englander, a Scottish as a Scotlander. so a person from United Kingdom can be referred to as "United Kingdomer"
Synonyms for United Kingdomer
On this page, you will find 30 slang terms related to United Kingdom's. Some of the top words include: Olde America, United Kingdom of Great Britain and Northern Ireland, scurvy, limbly, pooves, and 25 more.