I have the (Virgin)50mbps modem (call me old-fashioned), and everything seems to run fine with the connection very rarely timing out. A speed that is double that should be flawless, unless you live in the middle of nowhere.
You get exactly what is offered. 100 is 100!
Also, free upgrades are being given out early next year,
If you get 100, it'll be boosted to 150. If you get 200, it'll get boosted to 250
Maybe the sky broadband isn't good in my area? I recently read somewhere that ISPs are now legally required to meet certain speeds.
Virgin use the cable laid by the old Telewest at huge expense. It was bought out by NTL and then Virgin. Despite being laid in the late 80s and 90s it is much newer than the very old telephone cable everyone else has to use. I think we are on a 50mbs package and right now I am getting 42 which is pretty good. Certainly better than the half speed you are getting.