SMaSh = Single Malt and Single Hop. It is a movement of making great beer with simple grain and hop bills, and utilizing different hop timings, mash temps, etc., to achieve a multidimensional beer with simple ingredients.
Also, you dont need to keep to style. English IPAs, when compared to American, are slightly lower gravity, a little less bold in terms of hop flavor and aroma, and slightly dirtier/spicier hops used. And, of course, British Ale yeast is used, whcih will give some esters and flavors that an American Ale yeast would not. But you can mix up the hops in any way that seems exciting to you. Just remember what they are like, and decide what you want to go for. Cascade and Centennial are both American variety hops, and they have citrus and grapefruity aromas and flavors. Centennial is sometimes called "Super Cascade" because it has a very similar flavor profile to cascade, only bigger. Fuggles has an earthier, maybe spicier flavor, though it has some hints of fruit in there too, and it is a British hop. (I thought that it was a cultivar of East Kent Golding but grown in a different location, but I cannot find anything to support that so maybe I imagined it). And finally, Saaz is a Czech hop, spicy, earthy and mild, and traditionally used in German and Czech beers, as Beerrific pointed out, but also in US Wheat beers, Bitters and Belgian style beers. So if you wanted to you could experiment using this with the Fuggles, as they might compliment each other.
Also, remember that whatever you choose to use as your bittering hop, while it will contribute some flavor, will not contribute an excessive amount of flavor, and it could be another opportunity to include a hint of something different.
One idea might be to use Centennial as your bittering hop, as it has the highest AA and therefore you would need the least aount of it to get the IBUs and therefore less flavor imparted, use Cascade as your flavor and aroma hop, and then after fermentation is done split it to two different secondaries, dry hopping one of them with Centennial and the other with Fuggles...
Good luck, experiment and have fun!
