Like many finance websites I expect your Turkish bank will not allow you to access your account from a UK or any 'foreign' IP Address. It alsmost certainly works the same way in reverse, wherby you cannot access your UK bank account from a Turkish or any foreign IP address.
So either you need a VPN to mask the UK IP address as a Turkish one, or perhaps your Turkish bank will give you permission to access their account using a UK IP address, but you will almost certainly then need to provide a 'fixed' UK IP address. Which firstly means you are resticted to one place in the UK from where to access your Turkish account, and secondly probably will require some setup with your Internet Service provider (ISP) and possibly some changes to your home (W)LAN to accomodate the fixed IP address.