Of course the 4 UK associations are autonomous as their autonomy has been recognized in the FIFA statutes.
Common usage of the term "country" to refer to England, Scotland, Wales and Northern Ireland does not cut the mustard in regards to what FIFA define as a "country" 'it shall refer to an independent state recognised by the international community.'
One British nationality for 4 associations does not meet that definition.
Therefore there is a ruling inserted into the statutes
'Each of the four British Associations is recognised as a separate Member of FIFA.