If you're not going to use Euros again as it will normally be best to keep them why not sell them to friends at the current advestised rate and you'll all be quids in. Take it as it stands today with the US dollar, I'm using that as an example as I'm buying some tomorrow. The advertised rate for a pound is around $1.64, the Post office sell rate is $1.57 and their buy rate is $1.71. Now if I sell $100 to the post office I will get £58.47, if I buy $100 from the PO it will cost me £63.69.
Now let's say I have $100 dollars I want to get rid of I can do one of two things, sell it to the PO at their buy rate or to a friend at the POs sell rate, My friend is happy as he has his money at the regular high street rate, I am happy as I have passed the $100 dollars on at probably a similar rate, or may be, if I'm lucky, at a better rate than I bought them and have an extra £5.52 in my pocket.
One final bit of advice I always give is never use the High Street banks or larger high street companies for your currency, use the smaller travel agents or money shops and be prepared to barter. Buy the maximum amount you want as the more money you want the higher rate you can demand. Do your homework and know what the rates are in town, look on the internet for the online exchange rates and pitch yourself somewhere in the middle. The two rates will often be 4 cents apart so I can assure you if you are going to change up £500 in to US dollars you should get on the high street 2 cents higher than their advertised sell rate. Two cents on that £500 will put another $10 in my pocket.