Logitech are generally good, so just browse the Argos catalogue for one in the right price range.
High-end ones have a glass lens, as opposed to plastic, but for a webcam, plastic usually is fine. Then you can get ones with motion detection software etc.; mostly unrequired for basic uses.
Tip: use Skype over something else like msn. If you do plan on using Skype, then there are recommended webcams on their website: