Yeah. It's obviously incorrect in the sense celsius has no meaning as an absolute temperature scale and so its not 30% more anything, but in terms of colloquial

Yeah. It's obviously incorrect in the sense celsius has no meaning as an absolute temperature scale and so its not 30% more anything, but in terms of colloquial meaning the average Brit probably does see it as ~30% further along an indoor temperature scale from "someone put the heating on please" to "crikey, it's sweltering in here"