Originally Posted by Ghrumpy

I'm not up to date on your project, but are you saying the Batavus has a Swiss thread shell with a British BB in it right now? That's understandable, but I'm a little confused as to why a Dutch bike would have a Swiss thread BB shell.
Swiss thread is a little larger diameter than British, and a different pitch. They are not interchangeable. But the smaller diameter British thread can be forced into a Swiss thread shell. Shouldn't be, but often is when you have to made do, because of the rarity of Swiss thread BBs.

No offense intended, but I'm assuming you understand that both British and Swiss thread BBs have a LH thread on the drive side cup, and it removes righty-loosey (turning clockwise.)

If you're turning it the right direction, I would suggest some heating up of the BB shell with a heat gun or even a blow dryer. Not enough to burn the paint, but enough to make it uncomfortable to touch. That usually helps unless the BB cups are aluminum.