I assume you're ok with the major cities / attraction. Some other interesting places that I've come across: -
Glastonbury Tor, UK - while you're there, check out the Abbey (King Arthurs is buried there) and Chalice Well. If you have a car, Glastonbury is a good base for exploring the region - Stonehenge, Wells, Bath, the coast, Exmoor, Cheddar etc.
The Red Lion Pub, Avebury - the only pub inside a stone circle
Arbor Low, Peak District, UK - its a stone circle in the middle of some guy's farm.
Nine Ladies Stone Circle, Peak District - very photogenic at sunset. BTW, if you're heading to the Peak District, spend at least one night at Chatsworth House (ie Cavendish Hotel). Girls will be very impressed. In fact, spend a week there - its a good base for exploring the area and there are a lot of interesting and scenic walks, towns etc.
Tintagel, Cornwall, UK - King Arthur used to live there and its got a cool looking post office.
(If you're detecting a King Arthur, Stone circle theme, you're right - I've done several trips searching for the holy grail and my wife is a tarot card reader...)
The "Jurassic Coast", Uk - just watch out for dinosaurs!
The Abbey at Whitby, UK (and surrounding beaches) - great in the late afternoon.
York - if you like trains, vikings and walled cities, this is the place to be
Take a Boat Cruise on Loch Lomond, Scotland
There are a lot of waterfalls in Scotland...
Stirling Castle, Scotland - where you can get a great shot of the Wallace Monument. The staff are very good - If they don't offer, ask them to take your picture in the King / Queen's seat.
Basilica di San Miniato al Monte, Florence - a bit of a walk out of town, but you get a nice view.
Anywhere in Tuscany
Rome - my favourite place in Europe. If you're even half considering going to Europe and not visiting Rome, slap yourself a few times until you come to your senses.