I'm with Poguemahone on this one. By far the cheapest way is to tap the threads. It can be done! I had my LBS tap the Swiss BB with an Englishtap on my Peugeot fixie, then I forced two used English
cups into the threads several times for some extra oomph and then the guy who powdercoated my frame ran a tap through it again. It's dandy and running a Campy English BB right now. It all depends on the state the Swiss threading is in right now. If it is bad to begin with, don't do it and get the threadless BB made especially for shot BB threads.

But if the threads are fine, you want to convince someone at your LBS to run an English tap through it. Make sure he taps all the way to the end of the threads. This will likely scare a mechanic because the tap is not intended to force a new pitch into a shell and will consequently wear out faster, but the difference in pitch between Swiss and British is only 0.05 mm. Enough to prevent you from sticking a English BB in, but not enough for a tap to get stuck in.

I was a bit worried at the start of the project, but now I have a much wider choice of BB, both in size and quality!
hope this helps

cheers!