So I exceeded my 1GB monthly roaming data allowance on my TalkTalk plan by a few hundred Megabytes last month. I admit I wasn’t aware that data roaming wasn’t opt-in on TalkTalk, so got charged at standard roaming rates for my data overage in other European countries.
Compared to CHF 2.95/MB at Salt or or Sunrise (I think… their website currently fails to load), TalkTalk charged me a rather fair 0.01/MB as their standard roaming rate. 
That said, I’m still perplexed why one should use their “foreign packages” at CHF 8/500MB or CHF 14/1GB in the same zone 0 instead? 