I bought this one after getting the Mancuna. It seemed in a worse state, being quite siezed with rust, missing a handle, and generally looking sorry for itself. But then for £15 it seemed a shame not too.
Union, it seemed, were a brand name of TS Harrison from Heckmondwike in Yorkshire, established in 1898. Happily, the company itself is still going, though the "Union" brand name was dropped in the 1970s.
The drill is probably from the 1940s, the excellent Grace's Guide has this advert from 1945 for a "Union A1" drill, which was a 2-speed model, but otherwise looks quite similar to mine.
There's no model number or any other identification cast into the drill, apart from the name "Union" and "British Made".
As I said, it was completely siezed, it wouldn't turn at all, and none of the moving parts would move more than a couple of mm. So I gave it a good soaking in plus gas, which I repeated a couple of times a day, combined with some wiggling and fairly gentle tapping with a nylon hammer. After 3 days (!) of this, I finally got it to move enough that I could start taking it apart.
Doing so revealed that a couple of the teeth on the gear were broken (If you can make them out through the gunge and rust!). Unfortunately, these two were right next to each other, so would probably need fixing. In total there were 4 broken teeth.
Other than that, and the obvious rust, it all seemed to be in fairly good nick.