Gosh, not sure if to try to explain mild... I think that lots of Americans think it is a weak porter, but it's nothing of the like. Porters were heavily hopped beers that were aged and brewed with pale, brown and black malt. Milds were less hopped ales that were served unaged (mild) and brewed with pale malt. When milds became darker they did not become porters but they darkened by adding mainly dark brewing sugars. Checking Shut Up About Barclay Perkins, ABVs dropped through the 1800s from 6-10% abv to 4-6% by the 1910s, while SRM increased from 6-7 to 12+ in the same time, all with additions of invert sugar #2 and #3. Often some dark malts would be added but the roast flavours are not expected of a dark mild, with the focus being more on the dark fruit flavours. Dark milds can finish sweet or dry, but in general, even if they have (optional) roast flavours, these are secondary, making them completely unlike a porter, specially giving the low hopping and bittering.
If I had to describe some dark milds to a beer expert I'd say that they are more like half of a Belgian dubbel without Belgian yeast. They have half the IBUs, half the grain bill, they don't finish as dry and they don't have all the funks, esters and such like. They are not just bland brown beers. The mild I had last night was, flavour-wise mainly complex malts, cane sugar, a hint of coffee (but not roasty), prunes and raisins, a tad of bitterness but no hop flavour. There was also a clear note of brewery trademark yeast and the finish had a light dry/sour note. I'd describe a dubbel as the same lot of flavours but amplified, specially phenols and alcohol.
I bet now you're completely puzzled!
Not sure if you can find them on online shops, but I'd expect Thwaites Nutty Black (more of a roasty one), Cairns Dark Mild or Brains Dark to be more heavily distributed as they are regularly bottled and canned.