A bit more digging found a good photo, and further supports the claim that these are ferro-concrete hulled fuel barges from WW II:

https://www.flickr.com/photos/hamishfenton/9576708919

and the Imperial War Museum has a page about such vessels.

https://www.iwm.org.uk/collections/item/object/30018296

The ones I see in Google Maps, as well as the photo you posted, certainly seem to follow the IWM pattern.

I wonder if it’s a heritage of their use of coal for home heating. Good strategy to push it higher into the air so it gasses their neighbors instead.

They might also be disused fuel barges, dating from the WW II era. I found a couple (non-authoritative) references to "concrete fuel barges" littering the banks of the mouth of the Medway. They show up very clearly in Google Maps aerial view:

https://www.google.com/maps/place/River+Medway/@51.3942193,0.5698076,142m/data=!3m1!1e3!4m5!3m4!1s0x47df35fa0f3f5545:0x954645d0456cb547!8m2!3d51.2625867!4d0.348775

If that's what they are then there's ample precedent. The "Ghost Fleet of Mallows Bay" on the Chesapeake is an example: about 100 WW I era wooden-hulled steamers were scuttled at the end of the war, having never even seen active service because the war ended before they could be put to use.

https://en.wikipedia.org/wiki/Mallows_Bay#Ghost_fleet

A few more photos and an article can be found here:
https://savingplaces.org/places/ghost-fleet#.YrH5F0bMK70

and also here:
https://sometimes-interesting.com/ghost-fleet-of-mallows-bay/

Thanks, Bruce. I hoped someone would step up. I wondered if it was natural or maybe even unhealthy in some way. Interesting that most of the references fail to point out that it’s actually a maple.

I think Britain is the chimney capital of the world!

It's a maple but in Britain it's called a purple sycamore. It's not native but because it's been in Britain for hundreds of years it has become naturalized. I've seen these in Portland too. The leaves are interesting in that the underside color has quite a range of reds and purples. And the top side of the leaves is a deep, rich, darker green than other maples. https://henriettapark.org/658-2/

Well, yes. You make a good point. We did note that appreciatively. We’ve definitely seen worse.

But way better than the thing being narrow at the bottom and wide at the top!

Here's hoping for better days ahead. In my limited experience, the closer you are to London, the more difficult cycling is. So as you head north, I would expect things to improve. Beautiful pictures, as usual!

Interesting idea. They could easily be that old, and they have an institutional look. They look like pontoon boats to me.

Perhaps they are leftover prototypes from the British experiments to create artificial harbors on/after D-Day? I know they tried several designs, all of which involved basically chaining / cabling a bunch of floating objects together end-to-end and deliberately bottoming them to create calmer/slack water inside the enclosed space. Gaps at specific points made entrances and exits to the protected area.

That's only a guess, of course, and I'm probably wrong.

Oh, you’re right. I hadn’t even noticed. We’re getting a lot of daylight hours for our buck over here.

Hopefully yesterday was an exception, but we’ll see. This is as close as we’ll get to London so we should start seeing more open country again soon. You’re right about the variability of the NCN routes though. And I doubt that it’s changed much since you were here, other than to be made poorer by the stupidity of the Brexit decision. We’re not seeing any of the signs of new cycling infrastructure investment like you see elsewhere in Western Europe.

Oh, it wasn’t awful by any standard. I must have made it sound worse than it was. We’ve just gotten very spoiled and like to whine.

What an awful day! Hope better riding is coming up soon. But I know you two are problem solvers and will come up with a good plan to get to terrain that's more suitable for cycling if things don't improve. I've got my fingers crossed!