It's typically pronounced D-RAM I assume because RAM is already it's own common acronym people were/are familiar with. DRAM is just a specific type of RAM, Dynamic RAM, hence D-RAM.

I'm also a British English speaker, and I almost always hear "D-RAM". It's been a while since I'm encountered someone using "Dram".

On the other hand, I've always said "Prom" instead of "P-ROM" (for programmable ROM). I've never considered why there is a difference.

I have the same credentials but I've never encountered anyone pronouncing it as dram. Only D-RAM and S-RAM etc. Although the bike brand SRAM is pronounced as sram, which is fun.

Out of curiosity, would you pronounce S-RAM as sram?