My point stands, that's clearly not a 'simple single change', and also comes with an additional cost over and above your tickets.
Well it is a single change - get off at one station, taken directly to the next, get on next train. And most taxi ranks are far closer to the mainline platforms than the tube lines.
There is an additional cost, but frankly if you're making a long distance journey it won't be cheap regardless - and the likelihood is taxis are being used *somewhere* on the journey in any case.