everywhere credit card. take turkish lira only for matchday near the stadium..
cheers.
there is one more thing about cash, just came to my mind yesterday. i will write a long post 2 weeks later, mentioning everything. but i can mention it now also.
ok,
there are beggars all over istanbul. most of them small kids. they are kind. they sell stuff. when you sit in the streets they may come to you. they are kind dont worry. you may ignore them and they will go.
but sometimes, for example, when i sit in a pub at Besiktas or Kadıkoy, those kids come. i always give some money ( 5liras, 10 liras). if you ignore them they go away; but sometimes they can be too sticky and disturb your conversations with your friends. so you may hold some turkish liras for that. dont give more than 5 -10 turkish liras.
or you may bring your GBP coins from england (which you do not use), carry it, and give GBP coins to them.
cheers.