Even burning natural gas in a peaker plant to generate electricity (nTh ≈ 0.6), and account for transmission losses(~0.9), and then using that electricity to run a heat pump (~2.5) - is still more efficient than natgas space heating

Why is electricity still ~5x as expensive as gas then? (in the U.K. at least)

A small fraction of the cost of electricity is fuel cost. A much larger fraction is staffing, maintenance, and capital costs amortization. Distribution and maintenance fees are largely covered by per kWh price. The turbine I mentioned to do that generation costs $xx,xxx,xxx and is generally only cost effective to run as a peaker so that cost has to be amortized over fewer hours per day. Unlike natural gas hot water heaters etc, the turbines can't be built by the tens of thousands using cheap manufacturing techniques.

Because generation is the cheapest part of the electrical grid. Distribution and "overhead" cause end-user electricity prices to jump 3-5x.