Somalia is part of the Arabian peninsula, otherwise why would the country be part of the Arabian forum which is like the E.U but for the Arabs.(with many differences between the two obviously). And you're wrong, there's much history between Somalia and the rest of the Arab world add to that the same main relegion which is Islam and how influential that is for having similar culture.
As for your Britain example, Britain doesn't have Arabic as a second does it? Its not part of the Arab league now is it? It doesn't have Islam as a main relegion does it? It doesn't have borders with other Arab countries does it?
Oh right I forgot