10m not many really. 80m population, say around 40m smartphones. Is that fair?. 25% isn't enough to make it effective really is it? has it been out there long?
It's probably pretty ineffective at those levels, but the overall take up is less important than the take up amongst people who are likely to have more social contacts, especially where those contacts are with people they barely know.
So, for example, for blokes in their 50's and 60's who have limited social contacts, and the vast majority of those contacts are with people they know pretty well,an app isn't that important. If they get infected, they'll be able to tell anyone doing a manual track and trace who they met and give them those peoples contact details.
Any app's more important for people who are looking, or likely to meet new people whose contact details they don't have.
So, an example would be
@ForeverBlue92 going out on a Friday evening looking for talent, or hair cutting skills, to persue. If a contact tracer rang him, as well as trying to keep out of earshot of his missus, he might not have contact info for the hairdresser or the desperate divorcee he fingered, but, the app might have recorded their meeting, so could quietly let people know they'd been close to someone who was infected.