If you are getting TV from Telewest then you are already digital and you need to make no changes for the digital switchover.
As puzzlesRus says, if you got rid of Telewest then all you is a Freeview box and you have digital.
Note that this applies to ALL TVs in your house, so if you have a TV in your bedroom, or kitchen, or kids room, which is NOT on Telewest, then you will need a Freeview box for them all when we switch over.
Also note any VHS player or DVD recorder you have has a built in TV tuner (it is what you use to record onto your VHS tape or DVD).
If this TV tuner is analogue it will NOT pick up a signal after the digital switch over so you not be able to record on VHS tapes or DVD disks (unless you record from a Telewest box).