One theory (still fresh and untested) I've developed is that London may have been settled because an Equinoxial allignment here actually corresponds not to an artificial allignment (ala Stonehenge and most other allignments in London and elsewhere) but to a geographical one, namely the two hills of London, Ludgate and Cornhill, with a natural spring. There are other geographical allignments like this but they mostly consist of only two features, which are often hills (which are big enough not to make this too surprising as a chance thing) but 3 seems rare and might have struck those who passed through that this was a special place and worth settling. The midpoint of that key geo-allignment being St Pauls, perhaps explaining why it became so important. The mundane river trade explanation probably came later and explains the success of the city rather than its original reason to be here. Well it's a thought, and quite an entertaining one.