The cheapest way is probably by using what the ad's are calling 'a box with a slot'. i.e. Top Up TV. Buy a Freeview box with a slot. Pay �20 connection fee and �7.99 a month thereafter. (You'll get all the Freeview TV & radio channels + the Discovery Channel and services like UKTV Gold): http://www.topuptv.com/
just check that they have freeview in your area - I checked on mine tonight because the anologue digital tv service will be switched off eventually but freeview is not available in my area at the moment and I was able to check on the analogue and it will be switched off in my area in 2010