Just swapping the two terms around does not mean the same thing. "Democratic socialism" is socialism. "Social democracy" is still capitalistic with an expanded role of the state. Go educate yourself.
The people who got most of the UK social programs started in the 1900s called themselves democratic socialists, not social democrats. I am not sure when the terminology you are describing was introduced, but it is not universal.
https://en.wikipedia.org/wiki/Fabian_Society