Discovery have to many channels in the UK as it is.
Maybe so, but the Quest brand targets FTA viewers, where as the others behind the paywall don't. The UKTV factual channels will increase their share of the market in the UK and will be a better fit for a broadcaster that largely focuses on factual entertainment.
Hopefully with the entertainment channels coming under the same banner, they'll be less duplication of scheduling across brands as they've done at UKTV. I can see Drama becoming more classic sitcom based if and when Yesterday moves to Discovery to ensure FTA promotion of Gold.