The main source of general advice on Digital Switch over:

http://www.digitaluk.co.uk/

Meridian goes digital (Bluebell Hill) in 2012

http://www.digitaluk.co.uk/when_do_i_switch/meridian

OPTIONS: Include one off payment services (like Freeview, Freesat, and Freesat from Sky) as well as subscription services (like Sky, Top Up TV, Virgin, BT Vision and others).  Pick the one that suits your interests and circumstances.  Service comparisons are a good place to start!:

http://www.digitaluk.co.uk/cost__and__options/service_comparison

Check if you are eligible for assistance - over 75 for example:

http://help.digitaluk.co.uk/display/4/kb/article.aspx?aid=5984

So, do you need to buy a new TV?  NO!  Even if you have a black and white TV without a SCART socket, there is a set-top box that will enable you to receive digital TV - like Freeview.  Have a look through the Frequently Asked Questions:

http://help.digitaluk.co.uk/display/4/kb/article.aspx?aid=6118