My dictionary says that ' ‘boundary’ is not used when you cross from one country to another,' but here in this sentence it is used in this way. Could you tell me if it's an exception? And why is the word 'frontier' in quotes?
'The map shows a boundary between England and Wales, but there was no 'frontier', no customs officeers, no armed guard.'
Also, England and Wales may be separate nations ("people sharing a common culture"), but they aren't separate countries ("self-governing political entities").
England and Wales are not sovereign states, but most British people (and United Kingdom - Wikipedia, the free encyclopedia) regard them as countries:
"The United Kingdom is a unitary state [...]. It is a country in its own rightand consists of four countries, England, Northern Ireland, Scotland and Wales."