Yes, the British effort spent milllions of pounds, used eight boilers and a turnbine, and managed to push the record up by maybe 10% or so. I think they would have done better to build a replica of the Stanley from way back, with improvements to the chasis and plant as permitted by modern materials, plus improved streamlining. I would just about bet that by doing that they could have raised the record far more than 10 percent.
Pollution is a tricky thing, if you combust at good high temperatures and allow it to complete fully you fully burn the hydrocarbons, so only get water and CO2, except that means that the temperatures are high enough to form oxides of nitrogen, which causes the photochemical smog that visitors to LA will remember. If you keep the temperatures lower you eliminate the NO2, at the expense of allowing more unburnt hydrocarbons. It is of course easier to keep control of the combustion if it is taking place continuously and in a not too confined space, as you can eliminate quenching. So steam or hot air engines have that advantage. Unfortunately they both need heat exchangers, which is an enormous disadvantage for a portable plant.
regards
John