The UK is 4 countries as someone has said. Scotland, England, Northern Ireland and Wales, so like everywhere else, each country has its own football team.
United Kingdom basically is these separate countries, united under one Kingdom.
Gets more complicated in that there is also ‘Great Britain’ which is the Island comprising England, Wales and Scotland, excluding Northern Ireland. This as formed in the 1707 Acts of Union when England (which also encompassed Wales at the time) joined with Scotland to form Great Britain in Union.
Yet more complication- for example in the Olympics we compete as ‘Team GB‘ so not as separate countries, and in this context ‘Team GB’ is ‘Team GB and Northern Ireland’. Not sure why they just don’t call it the United Kingdom team