How do you connect to the internet? If it's with a cable provider then, yes, you can have your landline phone disconnected. However, if you connect via a BT line, you'll still have to pay the line rental in order to maintain your broadband connection.
Remember that internet phones only work when you're connected to the internet. You'll either have to leave your PC switched on, and online, 24/7 (which isn't good for your electricity bills or for the planet) or boot it up, and go online, whenever you want to make an outgoing call (or expect an incoming one).
It would certainly make sense to have an 'ordinary' phone as well (although a mobile might do the job) because you can't use the Tesco internet phone to dial the emergency services (or when there's a power cut). Also remember that your old phone number won't work with your new phone.
Reviews:
http://www.techdigest.tv/2006/07/review_tesco_ in.html
http://www.vnunet.com/personal-computer-world/ hardware/2154283/tesco-internet-phone
http://www.ciao.co.uk/Reviews/tescointernetpho ne_com__6485667
http://www.pcpro.co.uk/reviews/94200/tesco-int ernet-phone-e337.html
Chris