I’m with ID (which is Carphone Warehouse using 3’s network).
I asked about 2 years ago when I was looking, everyone said the same (they have caps etc) the problem is that the all pretty much work the same, they send an SMS to the actual phone saying “you’ve run out of data, reply with xxx to buy more”.
IF, you’ve got the kind of teen who will think “drat, I’m out of data, I’ll have to wait till I get home, I’ll be more careful next month” then the world’s your lobster.
But IF you’ve got the kind of teen who will think “We’ll it’s only £15 for another 1GB and this INSTA is the MOST IMPORTANT THING IN THE WORLD EVA and anyway I can claim I didn’t do it, or I must have done it by mistake or I’ll tell him my mate did it when I wasn’t looking” and repeat every day, or even several times a day never once stopping to count or consider how much all these texts were costing then don’t come crying when the bill hits £1000 (like it did for a friend of mine, TWICE).
I’ve got the latter, so I chose ID, you have to give them an over-spend cap of £5 (lasts about 30 seconds) but they cannot go over it without accessing the ID portal and wild horses couldn’t drag the password for that from me.
Used them for a year, then upgraded him to an iPhone SE for an extra £6 a month, obvs, being the social media darlin’ that he is, he couldn’t be parted with it when he decided to go swimming and waded into the pool with it in his pocket “I wasn’t going in deep” and it was deader than a dead thing inside 3 months.