Gosh I'm looking online at a Brompton pic, and I can't see the issue, it looks like plenty of radial clearance (between valve stem and hub), unless it's laced with spokes crossing the valve stem? Or do you mean circumferential clearance (along rim) to adjacent spokes? Is the pump head that large? Those valve extensions are designed for truck and trailer tires, and may throw your wheel off balance, if left on.