Switching circuit and transformer can be quite regular, you should use a multplying rectifier (Cockcroft Walton Multiplier) to reduce the required AC voltage respectively the winding ratio.
I would use a standard high frequency transformer for an off-mains SMPS, reversing primary and secondary.
With a multiplying rectifier, the switching operation mode is always summing forward and flyback. Above a minimum output voltage, the control behaviour is almost similar to a flyback converter.
TV HV transformers and cascades are designed for 12 to 25 kV and less current. They are also larger than necessary for 2.5 kV/25W supply.
can I use full bridge for this? I want to use L6203 to drive my power and with this IC i would not control mosfet on/off and its work easily. what is your opinion about this?