Yet another sad CFIT event. Speculation and irrelevant aircraft tracks don't help.
The flying conditions were not VFR
Conditions are VMC. VFR are Rules!
which suggests a sudden event rather than a developing problem
CFIT usually is! Witnesses reported it flew into a tree.
the experienced instructor would have been very familiar with the area
Familiarity does not help if you can't see.
as ATC was tracking them they will provide the answer.
But ATC had lost contact with them at 1240!
Whilst you are waiting for more factual information you might like to read
CAA Safety Sense Leaflet 23 Para 6 describes circumstances which match this accident perfectly. There was an almost identical event last January on the Chiltern Hills.