Before you buy an aerial, check the coverage predictor by inserting your postcode here:
Digital channels:
http://www.wolfbane.com/cgi-bin/tvd.exe?
Analogue channels:
http://www.wolfbane.com/cgi-bin/tva.exe?
It will tell you what type of aerial you need - look in the 'antenna suggestion' box at the right hand end of the table of results.