Since most of you know little or nothing about caves, here's some general info.
"...Divers said the temperature in the cave was a about 26 degrees (79 F), with water dripping from the walls, meaning the children were unlikely to have experienced dehydration and hypothermia..."
I was surprised to learn that the temperature was so warm underground, is this cave part of a volcanic system or hot spring?
It's rain water which is warm.
You've heard of snow, haven't you? Thailand has a much higher average temperature than my neck of the woods so the rain in Thailand is warmer, on average, than rain in my neck of the woods but the rain isn't the only factor in how warm the cave is. Depending on how extensive a cave is and how many entrances it has the temperature in the cave tends to be close to the average temperature of the area the cave is in. Here in the northeast caves tend to be around 52ºF, but with a lot of air movement or water flow the temperature may be closer to ambient. 79º may be typical in Thailand, but that's not very warm when the humidity is about 100%, you're dressed in wet (cotton?) t-shirts and shorts, and you aren't eating.
When the cave was flooding the current would be moving into the cave.
Water was obviously flowing into the cave, but that doesn't mean it was flowing into the "entrance". A cave "entrance" is just an opening that's big enough for people to get in and out but there are always other openings. Water may first collect on the surface and then drain into the cave in many specific locations that may or may not serve as an entrance for explorers, but it also just seeps through the ground and enters through millions of tiny cracks in the bedrock. Long before there's a cave there's just water seeping through all of the tiny cracks in the rock. Given time and suitable geology the water enlarges the cracks and forms a cave. The water drains downhill (but can also move uphill as a result of hydrostatic pressure), and just like surface drainages, collects in ever larger streams. I'm under the impression that the entrance they used in this cave is in a downstream area and is, or is near, a major exit for water that enters from other locations.
Caves that are formed by water dissolving rock (there are a bunch of other ways that caves can form) usually have dendritic drainage patterns, but for various reasons caves can have multiple streams flowing to different exits. Higher water levels can cause one drainage to spill over into another, so water entering in a particular area may not always flow to the same exit, and over time some drainage routes may be abandoned as streamflow is pirated to a lower route. That means there are often multiple routes from one spot to another, although in some small or simple caves there may be just one route between the only entrance and the farthest extent of the cave. It's possible that there are multiple routes from the entrance to where the boys were but I gather that either nobody was aware of alternatives or any known alternatives were worse than the route that was used. If there are multiple known routes I'll guess that the route the boys followed was the "standard" route for people heading to that area, though at some point rising water may have forced them to follow a route other than what they had planned on. Caves "backflood" when water levels rise and effectively move upstream (and uphill) when a restriction can't pass water as fast as it flows in. The boys may have encountered water flowing toward them from further into the cave and/or they may have had water levels rising behind them. At any rate, what definitely didn't happen was water flowing into a single entrance that's higher than everything else, filling the cave from a single point.
I have heard mention from a few UK sources that these sorts of dives (sump dives) are a different beast to that which a lot of cave divers will recognise
I think cave divers tend to fall into one of two groups. Some start out as divers and then start exploring water-filled caves. That frequently happens because some of those caves are filled with very clear water (until you cause a silt-out) and may have very pretty formations. The other group consists of people who started out as "dry cavers" who wanted to explore beyond the point where the ceiling drops below the surface of the water in a cave that mostly doesn't require diving. Those places are sumps, and the goal is to get through the sump to find more air-filled passage. While sumps can be filled with very clear water it's often muddy even before you have a chance to stir up the silt. Compared to many caves in Florida and the Yucatan the passage dimensions in some sumps may be fairly small, effectively making it impossible not to stir up the silt. Also, a sump is just a low spot in a passage, so the water may be ponded rather than flowing. In that case there's no current to carry away the silt you've stirred up. I think the typical cave diver hopes they won't stir up silt and the water will stay fairly clear. Sump divers expect lousy, and even zero, viz. I've heard sump divers in the northeast US refer to a "Dacor tempered glass" dive, for that little bit of writing on old school masks that is sometimes the only thing they can see. That can be a normal cave dive for them because the dive is a means to an end. It's also common for sump divers to use a rule of sixths instead of a rule of thirds, because difficult dives and restrictions are routine.
Anyone interested in a good read about diving and cave exploration might enjoy this:
Beyond the Deep: The Deadly Descent into the World's Most Treacherous Cave: William Stone, Barbara am Ende, Monte Paulsen: 9780446527095: Amazon.com: Books