I'm fairly sure O2's list is longer for unlimited data abroad, with Travel Inclusive (~48 in Europe Zone (granted that's limited to 25GB but some people report no limit) and ~27 for Travel Inclusive Zone, 75 total) and O2 has regular Travel available for some destinations as well (I think that combined with EU roaming does over 100 countries) for £6/day
I asked a few months ago and was told that it worked on EE Three Vodafone and O2Looks reasonably priced but couldn’t see who they partner with here.