'Smart boxes' (and 'Smart TVs) simply use services like BBC iPlayer to feed programmes to your TV (instead of your computer or other internet-enabled device). Because of licensing agreements, different boxes offer different services. e.g. the NOW TV box offers BBC iPlayer and Demand 5 (but not the ITV catch-up services). Sky are almost giving it away (at £9.99 including postage) because they'd like you to use it for their pay-as-you-go sports or movie services but you don't have to use them:
https://shop.nowtv.com/?DCMP=KNC-Brand-
Some boxes offer access to Youtube, or to paid-for services, such as Netflix or BlinkBox but trying to find one which simply offers all available catch-up services seems to be close to impossible. Even more oddly, it's often cheaper to buy a Blu-ray player that's got 'Smart' functionality built into it, than to buy a standalone Smart box. See the products described as 'Smart' here:
http://www.richersounds.com/products/home-cinema/home-cinema-separates/blu-ray
(but check carefully as to what services each one offers).
As I stated above, trying to find any device that offers ALL catch-up services is close to impossible but this Smart TV does it
http://www.richersounds.com/product/tv---all/samsung/ue22f5400/sams-ue22f5400
so I really can't understand why other products only offer a limited range of catch-up services.