Firstly it looks like its going to be wet, so the best places will be buildings by the river, apartments and offices i.e. indoors. The trouble is they'll be in hot demand so its time to have contacts and pull in some favours. One organisation I know with offices by the Thames is having a family day - how cool is that.
Then remember its a long way from Putney Bridge to Cadogan Square where the Queen boards her boat, so if you're upriver you'll miss a lot, though there are big screens being set up.
Another problem is a lot of the good locations are off limits, like the bridges obviously, for security reasons. Then some places have been reserved for functions, like around the Tower:
Couple of things to think about are:
- is there shelter?
- what's the backdrop for photos?
- how crowded will it be?
I'd have thought somewhere with an icon like the Houses of Parliament or London Eye the other side of the river would be good but again that's likely to be crowded.
Maybe somewhere further down stream around the Millennium Bridge with the big poster, the Globe and the Shard as background?
Oh and there are lots of portaloos appearing on the bridges.