http://www.topuptv.co.uk/
E4, UK Gold, Discovery Channel, UK Style, Bloomberg, Discovery Home and Leisure, Turner Classic Movies, Cartoon Network and Boomerang - £7.99 per month, plus a £20 connection fee (£10 if you sign-up online).
Requires an old ITV/On-Digital box (as you need a card slot) - although they plan on bringing out CAMs for IDTVs soon, & presumably will do something to let Freeview boxes take a card.
Also "requires you are able to receive Channel 5" - so I guess it uses the same MUX as Five does.
Hmm. Interesting idea. Wonder if it will take off...