Not sure about the others, but my £7.50 for 5Gb with 3 has a tit for tat arrangement with some countries, so my data usage is free.
It is a really odd list, not much in Europe but does cover Australia
"If you visit another country where we have a 3 sister network, you can use your price plan allowance to call and text home. It’s called 3 Like Home and when you run out of your allowance, you pay UK rates. So if you’re off to Australia, Austria, Denmark, Hong Kong, Ireland, Italy or Sweden, you’re in luck. "
Ouch for Monaco £6 per Gb mobile data
