i would say they are open all day staday.get there after 10am.most tabacs in spain have an exchange rate.so they will change it over first.then pay of course in euro;s.

tabacs follow the old spanish practice of opening morning and early evening and closing for siesta. there opening hours do change with the seasons but you can normally expect that their winter opening would be around 10-1 and 4-7. note that they are normally closed on sunday.
most tabacs are happy to take sterling, and the exchange rate is usually very good. it is normally better to pay in cash than to use a credit card.
prices are fixed in spain, see here
http://eurochange.es/seccion/en/prices/2/_table