sorry dont think you are right bbc at 320x256 was 20kb thats 4 colours nula is 16 thats 4bits per pixel 40kb video ram theyve doubled it no?
ula+ is 12kb video ram but the full 16kb suffers contention and also the remaining 4kb has some system variables from the basic rom and the interface 1 rom too no? so theres no xtra ram in the ula+ ? but spectra adds 32kb no? u sure its impossible to add hardware interlacing? would that double the vertical resolution were it possible?
have you seen the atari running zx spectrum software? think she needs the rapidus similar 2 the c64s super cpu