My heated throws (I have several of them bought over the years. One of them is more than 10 years old and still in perfect working condition) have two layers of soft fleece fabric with the wiring in the middle. The wires are attached to some type of straps that hold them in place. I actually pulled the wires out of an old one when it stopped working (after about 15 years so I got excellent use out of it) and gave the fleece throw to my dog.
The weight varies from one brand and style to another. My green sunbeam
is thicker fleece so it has a bit more weight than the older ones had. The new one is made by Biddeford
and it has thinner fleece but it's still soft and well made. It is smaller than a single blanket so that would make it weigh less too. I have no personal experience with UK brands. I pay around $20 to $25 on sale. I won't buy at full price.
I use the throw over the back of my chair during the day and when it's quite cold, say freezing or below, I put it between a top sheet and a quilt and leave it on low. That keeps me nice and warm. The power cord connects to one end so you can choose to put it either closer to your head or down near your feet depending on where you prefer it. This new one says not to sleep directly on the throw, but to use it as a blanket. It also says that it will be more efficient used with another blanket on top rather than putting the heated throw on the top. Heat gets trapped in close to your body so you can use a lower temperature and still be nice and warm. The wires don't bother me once I get laid down. I do have to pay attention when I get out of bed to make sure I know where the power cord is. It's not a problem at all.
I think a heavy duvet would weigh a lot more than my summer quilt and heated throw. I live in Michigan and we have a much colder and longer winter than you do in the UK. My pain would be much higher level and require more medicine without these heated throws.