The other day, when I saw whole Top Gear back catalogue on iPlayer, I watched the first boat challenge. One of comment I thought was strange is that Hammond said the camper van had 70 bhp when new, it must now have 40 bhp. (not sure about exact numbers)
I had zero interest in ICE so not sure, is this reduction in engine power true? How does this affect fuel consumption during use?
It's untrue. Certainly in the 80s and earlier engine power and efficiency could deteriorate with mileage, but not to the extent stated. There were multiple reasons for this:
Fuel - addatives to keep the engine internals clean were not as advanced.
Ignition - in the days of points and condenser ignition before full engine management the ignition timing and dwell angle could both drift, resulting in decrease in engine performance.
Carburation - before fuel injection petrol and air were mixed in a carburetor. These used a variety of methods to deliver a mixute that was close to the correct ratio at any given combination of engine revs, throttle drepression and engine load. For mechanical devices that work primarily on vacuum they were quite an achievement, but they were never perfect.
All of this would lead to a build up of carbon deposits inside the engine which would cause other problems (pinking, pre-ignition), as well as the mechanical components in both the ignition and fueling systems wearing and meaning they either require adjustment or replacement to restore performance and efficiency. Things would have to be really bad to see more than a 10% decrease in peak power though, and it was resolvable with some adjustments and possibly a "decoke" (a once in 100,000 miles procedure requiring removal of the cylinder head and some labour to scrape off the deposits and reseat the valves, I did a few when I was young) The cost of parts for this was small (I mean sub £50),
Nowadays with engine management and O2 sensors in the exhaust gasses you don't get carbon buildups as the engines always run at stoich and there are no moving parts in the ignition system and the only moving part in the fuel system is the throttle butterfly. Performance decrease with age is largely a thing of the past until you're talking interplanetary mileages, unless there's a design fault (e.g. the chocolate camshafts in some of the Audi V6 PD engines that would need replacing after about 200,000 miles).