To follow up, I’ve queried this on Reddit as well (sort by new):
I work for Her Majesty’s Passport Office – frontline customer service (HMPO). AMA from AMA
The advice there suggests cancellation should be prompt once my withdrawal letter is scanned in, though again I would be grateful for validation of this from other people.
Frustrating that there’s no automatic way to withdraw an application though I do get the potential reasoning (to avoid people sending on their supporting docs and then having no application that it’s linked to). I pretty much immediately realised my mistake but was unable to do anything about it.