The official exchange rate is ￡4.27. I checked the Post Office exchange rate, and they would actually give you ￡3.76 after they have taken their commission.
Right now at this very moment your 5 Euro equals 4.26 British Pound Sterling. Remember that the value of money fluctuates constantly which is based on world event. In short this is called Forex or Foreign Exchange. Have a look http://forex-bid.blogspot.com/ then you can convert all the money you find in the future.
Today (2012-04-12) it's ￡4.26.
The exact value fluctuates. Check this link for an up-to-date value: http://www.google.com/search?q=5+EUR+in+...
The exchange rate is ￡4.26 according to http://www.xe.com/currencyconverter/
but the tourist exchange rate would be lower so you would actually get less.
Roughly ￡2.50
Just found 5 euros and wondering how much it is in pounds hehe
