I think you mean IST (Istanbul). Technically BA are only able to accept credit cards issued in the country they are being used in e.g. Polish CC's in WAW, Turkish CC's in IST and British CC's in the UK. Some people report success in getting BA to accept UK cards in other countries but it soes vary. BA in Polans categorically stated I could not pay by CC and would have to send funds by International Money Order to their Bank. As messy and long winded procedure I was not happy about.

As Switch is now Maestro you may be able to use it.

The cost difference between IST and WAW is not that great. The problem with starting in IST is you are prevented from going to any Middle East country as IST is in the "no more than two flights between London and XXXXXXXX" rule. DXB, MCT, BAH, CAI are all good value long haul points and miles routes that would be denied to you by starting in IST.