Bathroom Mirror Demister

You are correct that the use of a 1.5mm T&E cable to power a mirror demister that is connected to a smart relay, which is in turn powered from a 2.5mm ring main without a spur, may not comply with UK electrical regulations. According to BS 7671:2018 (the IET Wiring Regulations), when using a 1.5mm T&E cable to supply a load that exceeds 1.2kW, a 5A fused connection unit or spur should be used to protect the cable from overload. This is because a 1.5mm T&E cable is rated for a maximum current of 16A, but the load presented by the mirror demister could exceed this rating. In addition, the use of a smart relay to control the power to the mirror demister may also require additional consideration. The wiring and installation of the relay should comply with relevant safety standards and regulations, such as those set out in BS 7671:2018. It's important to have the electrical installation checked by a qualified electrician to ensure that it complies with all relevant regulations and is safe for use. If any remedial work is required, the electrician can advise on the best course of action and carry out any necessary repairs or upgrades.

correct.

The load for the demister is probably tiny and bathroom cabinet lights are often connected to the light circuit using 1mm. The advice regarding 2.5mm2 for unfused spurs and 1.5mm2, if fused, is true for socket outlets. With this application, the load is far less than the 16amp capacity of the cable. I believe the other answer is likely produced by AI considering the answer structure and lack of context.