I use M&S Heatgen too. I love them because they just look like scoop neck, long sleeve t-shirts - depending on the weather I wear them under a thin cardigan, a thick jumper or just a coat. The colours are great - I've got them in blues, teal, red, purple, black and white (bought over three years).
To think that London, or anywhere else for that matter, does not belong to any one demographic