These may be fitted in your ensuite bathroom and downstairs WC.
Rather than take heat from your boiler, these have an internal heating element and are wired directly into the mains electricity.
You may find a fused switch next to your towel rail, or alternatively the switch may be outside the bathroom.
David Wilson homes are also often fitted with plumbed-in heated towel rails. You can identify these, as they will usually be fitted with a valve like a radiator and won’t have any wiring.