Looks like Virgin Media has added more episodes of both Lost and Brothers and Sisters in High Definition to its Video On Demand listings. You still have to hunt around for Lost HD (it's not under 4OD or High Definition, but only appears in the A-Z listing for TV Choice on Demand) but they've now added the missing episode and you get 1 -13 inclusive, all free.
Brothers and Sisters season one is where you'd expect it: under High Definition, Pay TV. They've added the missing episode and it now runs through to the end of the season, episodes 13 - 23. 99p each. The earlier episodes are similarly priced in Standard Definition.