why Britain out from the currency union?

Europe is connected with infrastructures like railways, telecom, etc. They have also made currency union, a prerequisite for political union. But Britain did not agree on the currency union. Britain could, at least, be beneficial from FOREX disadvantage. What is the reason Britain did not like to be part of the union.

All European countries have had economic difference when the form the currency union but given that the formed the union. Although, I have no deep knowledge on monetary economy, I don’t, however, see any advantage for Britain for being out of the union. Major reasons for the formation unity are barriers in infrastructures but Europe fulfils this.

The united Europe could be the learning and exemplary for the unity of other regions like Africa and finally the united world. Why not? But Britain for not accepting the simple currency union may delay the political union of Europe and would not be good example for nations in other regions.