IO540:
Whilst METARS are nice to have, they are (by definition) history.
For flight planning purposes it's the TAFs we should be looking at. The accident happened at about 1046Z. He therefore could not possibly have got to Shoreham in a PA28 before 1115Z which puts him into the:
BCMG 1113 7000 NSW.
Now 7km with No Significant Weather should have been well within his capabilities I would have thought.
The AAIB obviously considered the Shoreham METARS to have been irrelevent to the investigation.
They were obviously also irrelevent to the subject pilot for there is no record of him having asked for the Shoreham actuals and he was too far away and too low to pick up the ATIS.