I doubt Kate has gravity hot water. It is more than likely fully pumped with either a single pipe flow return like mine is or seperate flow and return pipes running around the house. I have all TRV's and no bypass because it is a single pipe system.
It all depends on the plumber at the time and 35 years ago when it was first installed it could have been done a few ways. My hot water coil comes of my single pipe just like a radiator does. I fitted a 2 port valve and thermostat to the cylinder so I could run the boiler at a high temp to heat the house without the water being dangerously hot for the kids.
Looking at my heating circuit i think Stevie Wonder did it.
I actually think the hot water was originally heated by a back boiler behind a coal fire and modified several times long the way. I have 1/2" 3/4" 15mm and 22mm copper in the house and a small amount of 15mm plastic.
Do I want to rip it all out and start again so it works properly - Yes. Do I want to remove floorboards and carpets in every room with a wife and 3 kids running about - No thanks. So it will stay like it is for now
