I use an ee data only sim in my ipad, which I buy from argos. Costs me £50. for 120 gb and is valid for a year.
That is adequate data for my purposes as it is only used when we are out and about. However, you do have to watch ee as they are inclined, after a few months, to shut it down. First time this happened they said it was because I had not topped up the data. I very politely reminded them it was valid for a year with half the data still there. They quickly switched it back on. This happened again on my second card, but got an apology for that one.
Still makes it the best value around and surely about the cheapest with good ee coverage.
Yes, but I have only been able to buy it from Argos. To be honest, I found it in their website and did not bother to look any further.
I should add, perhaps, I have previously used 30 day cards from O2, and it has always been a pain getting them to work. The ee card worked as soon as it went in.
Just make sure your tablet is not locked to one supplier. My ipad came from apple shop so no problem with that.