do kilts keep one warm enough in winter...?
Yes (within reason); a heavy weight kilt and a set of proper hose (I've a set of MoD Black Watch diced hose; 3 times as thick as some of my other kilt hose), with a waistcoat, jacket, and heavy wool Inverness coat would be quite enough for a London winter.

Someone asked me that the other day (a frequent question this time of year); I pointed out all the wool I was wrapped up in, and that they were the ones shivering in cotton blue jeans and a sweat shirt.