I'm not surprised at all that former Coronation Street writer RTD went for a Corrie cast member. He's done that several times. Heck, so has Chibnall. From
https://tardis.fandom.com/wiki/Coronation_Street:
And of course there's a great big Red Dwarf connection, as Craig Charles was a regular on Corrie for a few years, which led to a sort of crossover in Red Dwarf: Back to Earth.
For the benefit of Americans and Canadians who've never seen Coronation Street, it's not like American soaps. It started basically as part of the British kitchen sink drama trend, focusing on working class northerners instead of posh Londoners. I've been watching it for ages because my wife was a big fan and the woman in my life before her made me watch it with her at least once, so I figured I might as well give in. Like Doctor Who and a lot of other British things, it's noticeably more popular here in Canada than in the US. It can have some over the top storylines, and it's amazing how many characters have affairs or end up in prison at some point, but it can also be laugh-out-loud funny pretty often, and occasionally quite moving, like with the death of Julie Hesmondhalgh's character, and for that matter the early days of her relationship with the character Roy Cropper (Hesmondhalgh's character Hayley was a sympathetic trans character on a popular TV series more than twenty years ago, though Hesmondhalgh herself is cis female.) They also have fun referring to other shows; the shot of Millie Gibson cosplaying is from an episode where a younger character wants a Star Trek-themed birthday party, but they've had lots of Doctor Who references. Heck, Mary Tamm (Romana) made a one-shot appearance as a woman who walks into the corner shop looking for the latest issue of Canine Weekly (think about it), years after she played another character on the show.
As for Millie Gibson, her character is the orphaned teenage daughter of a gangster, who ends up being cared for by the man who killed her father (he's usually a good guy until they need an edgier storyline). Her character grows quite a bit over the years, so Gibson's played pretty much every kind of emotional state. That may be part of the reason soap actors are popular with Doctor Who producers. They've demonstrated they can play comic scenes and much heavier material.