Use cable able to stand higher heat. Ali-tube or other XLPE (Cross-Linked Polyethylene) or LSZH (Low Smoke Zero Halogen) cable is likely enough as hot water should not exceed 55°C as it normally is delivered through a metal tap and 423.1 limits the temperature of hand held part to 55°C with central heating since most radiators are metal the temperature is limited to 70°C as in most cases intended to be touched and maximum is 80°C as likely Metallic even if a part which need not be touched for normal operation. Since most central heating even if gas fired is controlled by electric it still comes under BS7671:2008.
The only exception would be where Myson type radiators are used so there is no access to hot parts.
Insulation is not really going to help as the cables still need cooling.
Although wrapping pipes with silver foil backed insulation like used to keep car inertia cool from summer sun will help.
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