ADDED: Confession: I had to Google to make sure "mash" referred to potatoes. The process of doing blog tags confronted me with the part of my brain — of whatever size IQ, I don't know — that has been dealing with English food. Long ago, "mash" got mushed with "mushy peas," and I'd never had the push to straighten that out until just now. But, thanks to Wikipedia, I've set my head straight. "Mash" is just mashed potatoes. And "bangers and mash" — which sounds dirty — is just sausages and mashed potatoes, which I think are "impressive."
The term "bangers" is attributed to the fact that sausages, particularly the kind made during World War II under rationing, were made with water so they were more likely to explode under high heat if not cooked carefully; modern sausages do not have this attribute.Wikipedia kindly includes some pop culture references, including Peter Sellers singing to Sophia Loren, about how he's unsatisfied with her "macaroni" and would like her to "give us a bash at the bangers and mash me mother used to make," which you can listen to on one of those YouTube videos where all you look at is the record spinning. Sellers and Loren do their best to exclude any double entendre that those lyrics may seem to convey in writing.
There's also a Radiohead number called "Bangers + Mash," which has lyrics that begin "You bit me, bit me, bit me, ow," which is something Peter Sellers never sang to Sophia Loren.
Times change, which reminds me of my favorite "banger" song: "Lieutenant Custard & His Banger of Time," which involves a time-travel sausage. Please be careful with this!