even the bbc model b has a 12bit 4096 colour palette now with 16 colours onscreen thats 40kb of video ram no?
https://github.com/bitshifters/bbc-nula/blob/master/readme.md
The BBC Micro VideoNuLA is a palette enhancer. In that respect it's a bit like ULAplus. It enables you to change the output of the default 16 colors to a selection from a palette of 4096 colors. Screens take up the same amount of RAM. But, the 6502 has memory addressed I/O, so there is no penalty for using a wide palette. The Z80 has 8-bit I/O, so using more than one byte per palette entry limits how many colors you can change on a palette line. The software HAM256 mode (which can display all 256 colors on screen) would not be possible with a wider palette. And ULAplus gives you 64 colors on screen from a palette of 256, which generally gives better results than 16 from 4096.
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