I'm surprised it's even 25% in the UK, and I wonder how that's measured. Maybe the 25% includes things like "opening the iMessage app to read SMS messages sent by two-factor authentication services" etc.
I’m not surprised, SMSing people is still common, and sliding from that to iMessage is totally frictionless for iPhone users. With iOS market share being around 50%, I’m surprised (skeptical even!) that it’s not closer to 50%.