17£/mo is sustainable when your users are just scrolling through Facebook and streaming Spotify. When they're all using it as their primary data connection and watching a few hours of HD Netflix a night, you need to charge more.

Unlimited mobile data went away or got expensive in most places right around the time that smartphones gained the ability to act as wifi access points.

The point is that without a way to tether your phone to a 'real computer', you have to be actively trying to use more than a couple of GB of data per month, so "unlimited" really means "up to maybe 5GB". While tethering was possible beforehand, it became much easier when wifi tethering was added to Android. This let anyone push a button and use their 3G / 4G data connection for torrenting, streaming and other such heavy duty usage, and suddenly "unlimited" phone plans were seeing hundreds of GB per month.

These days you can easily consume tons of HD video directly to your phone, even without tethering. So the situation has changed yet again.

"Incidentally", I suspect your plan was discontinued right about the time the number of competitors in the UK mobile market dropped from four to three. When 3 bought O2. And right about the peak of the 4g rollout. The same happened to my £13 plan. I'm not sure which of the above was the larger factor, or whether it was because usage patterns (tethering, Netflix) did in fact make these plans unsustainable. Interestingly, 3 also had their "one" plan (IIRC) which specifically allowed tethering, with unlimited data, for ~£20/mo.

Also, for completeness, it's probably worth mentioning the conditions of your unlimited data roaming were bandwidth limits, and a maximum trip duration of thirty days. Though I know people who considerably exceeded that duration in Europe and weren't noticeably restricted or penalised. Of course it's moot now (yay!- I write this message from Madrid airport..).