I think my experience runs counter to everyone elses. I was at Flamingo Land earlier in the week for a few days, couldn't even muster 1 bar of signal on Three using an iPhone 5S. I would have been ok with 2G but as we all know Three have virtually put the kybosh on that bar very remote locations. I am unsure of what impact MBNL may have had in the area, however although 3G signal was faint on EE, its 2G signal was healthy. Would have done me ok but alas I was on Three.
As for Vodafone and O2 at said theme park I got 2/3 bars of 4G yes 4G in the middle of nowhere. It was adequate for my needs, 2G coverage as you would expect from them was brilliant.