its better to use your bank card abroad if your bank supports this, the exchange rate is normally better then buying Euros...
Depending on where your going take sterling with you, you can get some really good deals abroad, but you cant guarentee what you get t'day you'll get t'morrow, an visa versa.
Always use my starling card abroad. No fees and you get the mastercard rate which is is usually better than you will get at a bureau. Failing that rob the post office, plenty in there.