1. MEF figures already allow for the 'unknown object' if there isn't a more significant one (such as a tall TV mast) already in the block.
2. If there is a known tall obstacle, the MEF is still correct as it is assumed that the 'unknown object' won't be placed on top of it!
Climbing to Safety Altitude is strictly only relevant in IMC. This was a VFR flight; the mast should be considered as a 'structure' - hence observe Rule 5 separation. I doubt whether Winter Hill is in a 'congested area', so the 500 ft rule applies if it's in an isolated location.
If you think you might enter cloud, 180 deg level turn and RTB.