I'm a bit late to chime in on this project, but you'll typically need at least an 800-1000V FET for the full 10% tolerance of 240V mains for flyback, single switch forward or push-pull. e.g. in flyback, there's the rectified input voltage plus the reflected output voltage plus a spike from leakage inductance. You can limit the spike with a voltage clamp, but not the rest. It depends how you design the flyback. Common practice is to desigh for a 50% duty cycle and use the turns ratio to convert the voltage, and if you do that, the reflected voltage will be twice the input voltage, so potentially 265*2^0.5=375V for the input voltage, multiplied by 2=750V plus the inductive spike. You can see that in the worst case high input voltage even an 800V FET won't be enough.
The good thing about the flyback converter is you don't have to work at the 50% duty cycle enen though it's common to do so. If you increase Ns/Np, you reduce the duty cycle for a given voltage, and reduce the reflected voltage which might help you use a lower voltage FET.
Forward converters with a reset winding can similarly be adjusted by adjusting the ratio of Np/Nr.
30W is possibly a bit much for a DCM flyback, but still doable, possibly with some relatively high peak currents.