UNESCO World Heritage Sites

I have a friend who has made it his goal to see some obscene percentage of UNESCO World Heritage Sites, and yet without trying I've managed to see more than him so far. So I figure if I'm to continue my lead I'm going to have to put some effort in to this. As far as I can tell, the sites in the UK are (according to wikitravel, a site that will probably pop up here frequently as I post):

United Kingdom

• Dorset and East Devon Coast, West England
• The Giant's Causeway in Northern Ireland
• Durham Castle and Cathedral
• Ironbridge Gorge near Telford
• Studley Royal Park near Ripon and Harrogate
• Stonehenge
• Caernarfon Castle, Conwy Castle, Beaumaris Castle & Harlech Castle in Wales
• Saint Kilda, Outer Hebrides
• Blenheim Palace near Oxford
• Bath
• Frontiers of the Roman Empire: Hadrian's Wall
• The Palace of Westminster
• The Tower of London
• Canterbury Cathedral
• Edinburgh, Old and New towns
• Gough Island Wildlife Reserve, a South Atlantic Island
• Maritime Greenwich
• Neolithic Orkney
• Blaenavon Industrial Landscape in Wales
• Derwent Valley Mills, Derbyshire
• New Lanark near Glasgow
• Saltaire, in Bradford
• Kew Gardens
• Liverpool – Maritime Mercantile City
• Cornwall and West Devon Mining Landscape

Now I'm not sure which of these are not to far off the path we'll be taking any way, nor which are actually worth seeing (not all UNESCO sites are created equal, after all). But I was hoping you all could chime in on sites you've visited and your impression.

Durham Cathedral I know we'll see. The Tower of London is the one I'd probably push for second on the list. Stonehenge, touristy as it must be, is probably something I should see if we're in the neighborhood (perhaps we can make it a middle of the night destination so there won't be people around to ruin it?). And that's where my knowledge ends. So what's the verdict? What's worth taking the time to see and what's not? Any interesting stories or histories around these places that I'm ridiculous not to know?