If you're worried about your mains cabling coming into contact with the insulation (and hence overheating) then I think one option is to over-specify the cable... i.e. use thicker cable than you'd normally use. The larger the cross-sectional area of the conductor, the lower the temperature increase for a given current. So a big, fat cable will heat up less than a thinner cable, for a given current. But yes, as Red Devil said, definitely check the most recent Part P regs. I think they're available for free online as a PDF download. I'm not a qualified electrician so please don't take my advice without checking first!