Your 1950 Clubman should have GB Sports brakes. As I recall (and it's been years now since I owned no fewer than three of these delightful machines) they do take the special Raleigh brake cables. So you could either source regular GB Sport calipers (hardly difficult on eBay UK) or, just as easily, score NOS Raleigh brake cables also on eBay UK. They come up frequently and I bet there are sets on offer this very minute. And I bought a pile of them a few years ago. Which reminds me to look to see if I may have a set remaining. Technically, you need the silver smooth housing but the more common grey ribbed version looks fine, too.
If you have a British bicycle, forget American eBay... troll the UK site instead and all manner of goodies, now competively priced given the value of Sterling against the Dollar (finally!) will materialise.
Peter Kohler
Washington DC USA