For a VFR flight from Biggin to Old Sarum, and given there is no serious terrain along the route, I would get the TAFs and (before the actual flight) METARs for airports along the route i.e.
Biggin
Gatwick
Farnborough
Southampton
Boscombe Down
The cloudbases given in these will be above each airport's elevation. One needs a good 1500ft between the cloudbase and the ground for a safe VFR flight.
One also needs a general idea of the conditions for a wider area and the MO form 215 is good for that.
Those are the basics, anyway.