Two thoughts: have you tried our Polish friends such as Legre, Piotross, 126Fan etc (see the For Sale threads) for good condition used parts, or Axel Gerstl? Axel Gerstl actually list this part for the a/c so maybe worth an email for any odd NOS Bis item they may have that's not a line item;
second thought is, what about getting your existing camshaft re-profiled? A quick google turns up
http://www.newman-cams.com/dealers.html Newman seem to have an agent near you in Eccleshall. If it's a bit too pricey for a one-off, how about exploring the possibility of getting a group order? There are many Bis owners on here, and you can't be the only person facing this problem. Again, just a phone call to Newman might give you a lot of useful information.
It might also be worth asking our European friends for pricing for re-profiling your own camshaft. I recall an instance where alloy wheel refurbishing was so differently priced between France and the UK that it was cheaper to send the wheels to the other country (iirc it was from France to UK) and back than to get the work done locally.
I just found Newman at random, but I'm sure there must be other similar firms around the country.