Well, I had one of these monsters
Which flipped open to reveal a keyboard for ONmail and other interactive applications ONdigital offered (and none of which I can remember using)
The onMail app could have been great, but given that many homes didn’t have an always-on internet connection and the ONdigital box had to dial up the internet (which it managed at something like 22kbps) it meant you had to check if you had mail, rather than receiving any sort of push notification. ONmail was apparently launched in March 2000, which seems quite late to me for a dial up text only email service. I think I used it a bit once or twice, then didn’t bother anymore, because if I was dialling up the internet for something, I’d check my emails then anyway.
ONrequest was a pay per view movies system, and I think I may have used that once or twice. Again, that needed to make a phone call to the internet to log the charge and you had to view at a certain time, so it wasn’t really on request at all.