'Upgrading' to skinny tyres pointless for London commuting

That's like saying you might as well ride old school until you've done under 50 minutes for 25, because you're never going to get comp record on any bike until you can do 30mph on 1970s tech.

As gains go, swapping from butyl to latex has to be about the cheapest in W/£

I kind of agree with this. People spunking all their money on the most tenuous of light & aero parts for marginal benefits when they are not even "competitive". Why not train and get good on any old thing which is practical and cheap and when you start getting impressive numbers buy fancy gear in prep for racing.

If someone is not even competing, but challenging themselves for fun. Then it would also be reasonable to get a period correct bike and try to match old records for TT's and such.