For ADSL information on particular ISPs and prices have a look at http://www.adslguide.org.uk/
Cable :
ntl: http://www.virginmedia.com/ / http://cablehell.co.uk/
tw blueyonder: http://www.blueyonder.co.uk/blueyonder/index.jsp
Tweaking your broadband speed : http://www.dslreports.com/tweaks
Feel free to post other sites that you feel may be helpful.
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