If the new wheels and tyres now fit the spare wheel well, and the old ones didn’t, then the new ones must be smaller. Hence speedometer will over-read. Be very mistrustful of anything on the web relating to ‘equivalent’ tyres & wheels, there’s an awful lot of dangerously inaccurate data published.
Also worth bearing in mind, no speedometer that is driven from wheel sensors can be very accurate, at least not all the time, simply because it will drift by a percent or two as the tyre tread wears down (less tread means smaller circumference). Legally they are allowed to over read a little, but must never under read.
I think satnavs can usually be trusted, I suspect that Sunday times article is just plain wrong about how Sat Navs work, in relation to speed measurement. I would assume they measure speed by detecting phase shifts and Doppler changes in the Satellite signals rather, than repeated location sampling.
As others have said, make sure your insurers know, else you might be accused of ‘non disclosure’ if you ever need to claim. They may want to increase your premium, the argument often is “different wheels might make the vehicle more attractive to a thief”.