1500 feet (at least that is what is used by the North Sea Helicopter companies). And to descend below that requires a CAA approved self interpreted radar approach.
It is not so much the fixed obstacles (although wind turbines are sprouting up daily) but the moving ones that are of concern. Most (but not all) vessels that you can expect in British waters will be below 500 feet high. But it is not uncommon for Mobile Offshore Drilling Units to be towed around the coasts of NW Europe and they will be around 500' high. There are also a couple of crane barges that visit the North Sea from time to time with 200m long booms. It is an amazing sight to see them sticking out of the top of sea fog.