Didn't they prove that Stonehenge was a calendar of sorts? Maybe I'm thinking of something else... As for the spiritual aspect of that place...
There's so many theories, but the only one that I even consider plausible would be rituals. Old spiritual rituals... probably involving the moon's phases...
(Examples of totally unreasonable theories: interdimensional portalways; Merlin's true home; a ressurection alter; etc.)
I've never heard of Avebury... so I've no opinion on that one... I'll go look it up... be right back...
*Six minutes later*
Wow... serpent and sun, huh? The site I went to says that's a traditional alchemical symbol. Interesting... I need more info...
This is supposed to be Avebury, and quite a bit of its surrounding land...

And this is just a drawing of the stones before many of them were removed...

Wow... This is really interesting... I need to look into this... That reminds me; I need to do more research on Stonehenge, too...
Sorry for rambling like this! .^_^;;.