As well as the take off and climb performance being affected by a density altitude higher than many would be used to in the UK climate, no-one has yet mentioned the higher liklihood of plug fouling, which could explain a loss of already somewhat limited power ..two fouled plugs in one cylinder can be hard to detect in cruise or on a mag check and may only show up as less than minimum static RPM on full power at take off.
The 140 is a lovely machine, but those little fat wings can quickly get you on the wrong side of the drag curve and keep you there if other things are already stacked againt you.
In my experience as a stude and instructor in the UK, (including flights to Sandown) and as an instructor here in Aus checking out UK pilots, the old adage "I was told NEVER to touch the mixture below 5000 feet!!!!"
in direct contrdiction of the POH seems to be taken as gospel...
Condolences to those involved and their loved ones.