The trouble with all this is many people in the UK out of the cities have poor upstream speeds like Analoguesat. A Slingbox is going to hog almost all of that. If you configure QoS (assuming that you have a router with this facility- many don't) to prioritise other traffic, a Slingbox's upstream will slow to a halt and your friend or relation abroad trying to watch Eastenders is going to get upset.
It's also not practical for many households to have a spare feed to a Freeview box or spare
Sky box, in some cases another aerial will have to be erected as Freeview signal strength is marginal in many areas and cannot be split.
It's also the case that many expats do not have houses back in the UK, or if they do, they are usually rented to tenants. Imagine how fed up tenants would be if the ADSL to the house was being slowed right down by their landlords using a Slingbox hidden away in a cupboard.
It's a neat concept if you are wealthy enough to have properties in two countries, and able to pay for broadband in both, but for the majority- it's a nice, impractical, idea.