I flew to a south coast airfield last Saturday (26th) and also got caught out by the fog... had to wait on the ground for an hour before the skies cleared.
When I took off, the surface visibility was around 2500m, with no cloud directly overhead, but there was fog (base ~ 300ft, tops less than 500ft) on the climb out. I reckoned it was a reasonably safe bet as the airfield itself was clear.
Generally, I'd like 1000ft clearance between the cloudbase and the ground en route, to give me a chance if the engine conks out.
BTW, as someone who has recently gained an IR... and is a bit rusty on the rules for departing VFR and then going IFR once airborne... what are the legal minima for a VFR departure? I believe it's 1800m visibility, but is there a cloudbase requirement as well?