Why a single market needs a single currency

Tonight I found out my debit card give me 75 BP for 100 Euros. Not as bad as 70 I got elsewhere but some Oxford St tourist shop give me 80 and a well known department store 82!

That means 750 vs 820 BP per 1000 Euros, or 70 BP per 1000 Euros or 7%. That's 1.333 Euros per BP. Compare with central rate!

That is why a real single market - space also needs a single currency. The rest is hot air. Europe, unite or let yourself decay!