Both sites have been looked at fairly well over the years - as Braveduck says, access is currently not good.
As far as Brants Gill Head is concerned, draining it out is probably a non starter. There are two sumps not far inside the entrance. The active sump to the left is low and immature. The origin of the water emerging from it is the "static" sump pool over to the right (which actually isn't static but has a small underwater bedding just below water level over to the left feeding the active sump, such that the "static" sump only overflows the gravel bank at the end nearest the cave entrance in high discharge). The point is that the levels of the two sumps are almost certainly the same; the active sump is floored by solid bedrock and it's likely that any excavation of the gravel mound which you pass over to reach the static sump would hit bedrock at the same level.
The "static" sump is the main way on and it leads down to a boulder choke at -6 m. You can see the way on down through the boulders, deeper still. This choke has been dug by cave divers in the past but it's a major project and there were better prospects elsewhere. In order to drop the water enough to dig this choke in the dry (then explore whatever is beyond) it would be necessary to lower the water level by up to 9 metres. But any such channel would need to be made through bedrock for tens of metres . . .
For some detail about the topography in the cave, both above and below water, see a report from 15 years ago in CDG Newsletter 148 pages 2-4 which includes a survey plan.
There is a story of this choke having been passed to a short length of streamway and a second sump (in the 1970s I think). The story goes that the route through the choke was so unstable there was a risk of it collapsing with people beyond. An ammo box of supplies was taken through to the streamway so any unplanned delay could be sat out whilst a new route through the choke was being excavated. Most CDG members of the day were sceptical of this story but, if anyone ever gets through the choke and surfaces, it would be interesting to look around for the rusty remains of the ammo box. I always thought it would be really nice if this ammo box were to be found one day, silencing all the doubters! (The person associated with this story sadly passed away a number of years ago, so it's not possible to ask for further detail.)
However, should the above story prove true, the existence of even a short section of streamway would mean that altering the water level inside Brants Gill Head would have no effect on the downstream sumps in the various feeder caves.
Douk Gill Cave has also been pushed quite hard over the years by a number of cavers. The left hand of the two entrances does actually draught but becomes too low after a low, wet area. My understanding is that the farmer there particularly does not want caver visits currently (but Braveduck will be more clued up than me on the present access situation at this particular site). There is one thing I've always wanted to look at there myself but I've stayed away for many years, in respect of the farmer's wishes. (After all, there's no shortage of other projects to pursue meanwhile.) It's on the list though, in case the access situation alters in future.
Hope that helps.
