Default Three UK Model - Flash to stock?

My Three model comes unlocked but is already behind when it comes to firmware. It also only has the APNs for Three (so I have to manually input for any other SIM I insert) and there's an annoying splash screen.

I'm not very well-read when it comes to flashing HTC devices but am I right in thinking that there is no way to flash stock firmware without S-off and bootloader unlock? One good thing about even the Sony handsets is that you can flash stock firmware without going through hell.

If I'm wrong could someone point me in the right direction. Not the end of the world if I can't 'stockify' but it would be nice